CHAPTER 3

Hackers' Equipment

You can hack with almost any microcomputer capable of talking to

the outside world via a serial port and a modem. In fact, you don't

even need a micro; my first hack was with a perfectly ordinary

viewdata terminal.

What follows in this chapter, therefore, is a description of the

elements of a system I like to think of as optimum for

straight-forward asynchronous ASCII and Baudot communications. What

is at issue is convenience as much as anything. With kit like this,

you will be able to get through most dial-up ports and into

packet-switching through a PAD -- a packet assembler/ disassembler

port. (It will not get you into IBM networks, because these use

different and incompatible protocols; we will return to the matter of

the IBM world in chapter 10.) In other words, given a bit of money, a

bit of knowledge, a bit of help from friends and a bit of luck, what

is described here is the sort of equipment most hackers have at their

command.

You will find few products on the market labelled 'for hackers';

you must select those items that appear to have 'legitimate' but

interesting functions and see if they can be bent to the hacker's

purposes. The various sections within this chapter highlight the sort

of facilities you need; before lashing out on some new software or

hardware, try to get hold of as much publicity and documentation

material as possible to see how adaptable the products are. In a few

cases, it is worth looking at the second-hand market, particularly

for modems, cables and test equipment.

Although it is by no means essential, an ability to solder a few

connections and scrabble among the circuit diagrams of 'official'

products often yield unexpectedly rewarding results.

The computer

Almost any popular microcomputer will do; hacking does not call

upon enormous reserves of computer power. Nearly everything you hack

will come to you in alphanumeric form, not graphics. The computer

you already have will almost certainly have the essential qualities.

However the very cheapest micros, like the ZX81, whilst usable,

require much more work on the part of the operator/hacker, and give

him far less in the way of instant facilities.

** Page 15

(In fact, as the ZX81 doesn't use ASCII internally, but a

Sinclair-developed variant; you will need a software or firmware fix

for that, before you even think of hooking it up to a modem.)

Most professional data services assume the user is viewing on an

80-column screen; ideally the hacker's computer should be capable of

doing that as well, otherwise the display will be full of awkward

line breaks. Terminal emulator software (see below) can some- times

provide a 'fix'.

One or two disc drives are pretty helpful, because you will want

to be able to save the results of your network adventures as quickly

and efficiently as possible. Most terminal emulators use the

computer's free memory (i.e. all that is not required to support the

operating system and the emulator software itself) as store for the

received data, but once the buffer is full, you will begin to lose

the earliest items. You can, of course, try to save to cassette, but

normally that is a slow and tedious process.

An alternative storage method is to save to a printer, printing

the received data stream not only to the computer screen, but also on

a dot matrix printer. However, most of the more popular (and cheaper)

printers do not work sufficiently fast. You may find you lose

characters at the beginning of each line. Moreover, if you print

everything in real-time, you'll include all your mistakes, false

starts etc., and in the process use masses of paper. So, if you can

save to disc regularly, you can review each hack afterwards at your

leisure and, using a screen editor or word processor, save or print

out only those items of real interest.

Serial ports

The computer must have a serial port, either called that or marked

RS232C (or its slight variant RS423), or V24, which is the official

designator of RS232C used outside the USA, though not often seen on

micros.

The very cheapest micros, like the ZX81, Spectrum, VIC20, do not

have RS232C ports, though add-on boards are available. Some of the

older personal computers, like the Apple or the original Pet, were

also originally sold without serial ports, though standard boards are

available for all of these.

You are probably aware that the RS232C standard has a large number

of variants, and that not all computers (or add-on boards) that claim

to have a RS232C port can actually talk into a modem.

Historically, RS232C/V24 is supposed to cover all aspects of

serial communication, including printers and dumb terminals as well

as computers. The RS232C standard specifies electrical and physical

requirements.

** Page 16

Everything is pumped through a 25-pin D-shaped connector, each pin

of which has some function in some implementation. But in most cases,

nearly all the pins are not used. In practice, only three connections

are essential for computer to modem communication:

Pin 7 signal ground

Pin 2 characters leaving the computer

Pin 3 characters arriving at the computer

The remaining connections are for such purposes as feeding power

to an external device, switching the external advice on or off,

exchanging status and timing signals, monitoring the state of the

line, and so forth. Some computers and their associated firmware

require one or other of these status signals to go 'high' or 'low' in

particular circumstances, or the program hangs. Check your

documentation if you have trouble.

Some RS232C implementations on microcomputers or add-on boards are

there simply to support printers with serial interfaces, but they can

often be modified to talk into modems. The critical two lines are

those serving Pins 2 and 3.

A computer serving a modem needs a cable in which Pin 2 on the

computer is linked to Pin 2 on the modem.

A computer serving a printer, etc, needs a cable in which Pin 3 on

the: computer is linked to Pin 2 on the printer and Pin 3 on the

printer is linked to Pin 2 on the computer.

If two computers are linked together directly, without a modem,

then Pin 2 on computer A must be linked to Pin 3 on computer B and

Pin 3 on computer B linked to Pin 2 on computer A: this arrangement

is sometimes called a 'null modem' or a 'null modem cable'.

There are historic explanations for these arrangements, depending

on who you think is sending and who is receiving--forget about them,

they are confusing. The above three cases are all you need to know

about in practice.

One difficulty that frequently arises with newer or portable

computers is that some manufacturers have abandoned the traditional

25-way D-connector, largely on the grounds of bulk, cost and

redundancy. Some European computer and peripheral companies favour

connectors based on the DIN series (invented in Germany), while

others use D-connectors with fewer pin-outs.

** Page 17

There is no standardisation. Even if you see two physically

similar connectors on two devices, regard them with suspicion. In

each case, you must determine the equivalents of:

Characters leaving computer (Pin 2)

Characters arriving at computer (Pin 3)

Signal ground (Pin 7)

You can usually set the speed of the port from the computer's

operating system and/or from Basic. There is no standard way of doing

this; you must check your handbook and manuals. Most RS232C ports can

handle the following speeds:

75, 110, 300, 600, 1200, 2400, 4800, 9600

and sometimes 50 and 19200 baud as well. These speeds are selectable

in hardware by appropriate wiring of a chip called a baud-rate

generator. Many modern computers let you select speed in hardware by

means of a DIL switch. The higher speeds are used either for driving

printers or for direct computer-to-computer or computer-to-peripheral

connections. The normal maximum speed for transmitting along phone

lines is 1200 baud.

Depending on how your computer has been set up, you may be able to

control the speed from the keyboard--a bit of firmware in the

computer will accept micro-instructions to flip transistor switches

controlling the wiring of the baud-rate generator. Alternatively,

the speeds may be set in pure software, the micro deciding at what

speed to feed information into the serial port.

In most popular micro implementations the RS232C cannot support

split-speed working (different speeds for receive and transmit). If

you set the port up for 1200 baud, it has to be 1200 receive and

transmit. This is a nuisance in Europe, where 75/1200 is in common

use both for viewdata systems and for some on-line services. The

usual way round is to have special terminal emulator software, which

requires the RS232C hardware to operate at 1200 /1200 and then slows

down (usually the micro's transmit path) to 75 baud in software by

means of a timing loop. An alternative method relies on a special

modem, which accepts data from the computer at 1200/1200 and then

performs the slowing-down to 75 baud in its own internal firmware.

Terminal emulators

We all need a quest in life. Sometimes I think mine is to search

for the perfect software package to make micros talk to the outside

world.

** Page 18

As in all such quests, the goal is occasionally approached but

never reached, if only because the process of the quest causes one to

redefine what one is looking for.

These items of software are sometimes called communications

packages, or asynchronous comms packages, and sometimes terminal

emulators, on the grounds that the software can make the micro appear

to be a variety of different computer terminals. Until recently, most

on-line computer services assumed that they were being examined

through 'dumb' terminals--simply a keyboard and a screen, with no

attendant processing or storage power (except perhaps a printer).

With the arrival of PCs all this is slowly changing, so that the

remote computer has to do no more than provide relatively raw data

and all the formatting and on-screen presentation is done by the

user's own computer. Terminal emulator software is a sort of

half-way house between 'dumb' terminals and PCs with considerable

local processing power.

Given the habit of manufacturers of mainframe and mini- computers

to make their products as incompatible with those of their

competitors as possible (to maximise their profits), many slight

variants on the 'dumb' computer terminal exist--hence the

availability of terminal emulators to provide, in one software

package, a way of mimicking all the popular types.

Basic software to get a computer to talk through its RS232C port,

and to take in data sent to it, is trivial. What the hacker needs is

software that will make his computer assume a number of different

personalities upon command, store data as it is collected, and print

it out.

Two philosophies of presenting such software to the user exist:

first, one which gives the naive user a simple menu which says, in

effect, 'press a key to connect to database' and then performs

everything smoothly, without distracting menus. Such programs need an

'install' procedure, which requires some knowledge, but most

'ordinary' users never see this. Normally, this is a philosophy of

software writing I very much admire: however, as a hacker you will

want the precise opposite. The second approach to terminal emulator

software allows you to re configure your computer as you go on--there

is plenty of on-screen help in the form of menus allowing you to turn

on and off local echo, set parity bits, show non-visible control

codes and so on. In a typical hack, you may have only vague

information about the target computer, and much of the fun is seeing

how quickly you can work out what the remote computer wants to 'see'

- and how to make your machine respond.

** Page 19

Given the numbers of popular computers on the market, and the

numbers of terminal emulators for each one, it is difficult to make a

series of specific recommendations. What follows there- fore, is a

list of the sort of facilities you should look for:

On-line help You must be able to change the software

characteristics while on-line--no separate 'install' routine. You

should be able to call up 'help' menus instantly, with simple

commands --while holding on to the line.

Text buffer - The received data should be capable of going into the

computer's free memory automatically so that you can view it later

off-line. The size of the buffer will depend on the amount of memory

left after the computer has used up the space required for its

operating system and the terminal software. If the terminal software

includes special graphics, as in Apple Visiterm or some of the ROM

packs used with the BBC, the buffer space may be relatively small.

The software should tell you how much buffer space you have used and

how much is left, at any time. A useful adjunct is an auto-save

facility which, when the buffer becomes full, stops the stream of

text from the host computer and automatically saves the buffer text

to disc. A number of associated software commands should let you turn

on and off the buffer store, clear it or, when off-line, view the

buffer. You should also be able to print the buffer to a 'line'

printer (dot-matrix or daisy wheel or thermal image). Some terminal

emulators even include a simple line editor, so that you can delete

or adjust the buffer before printing. (I use a terminal emulator

which saves text files in a form which can be accessed by my

word-processor and use that before printing out.)

Half/full Duplex (Echo On/Off) - Most remote services use an echoing

protocol: this means that when the user sends a character to the host

computer, the host immediately sends back the same character to the

user's computer, by way of confirmation. What the user sees on his

computer screen, therefore, has been generated, not locally by his

direct action on the keyboard, but remotely by the host computer.

(One effect of this is that there may sometimes be a perceptible

delay between keystroke and display of a letter, particularly if you

are using a packet-switched connection--if the telephone line is

noisy, the display may appear corrupt). This echoing protocol is

known as full duplex, because both the user's computer and the host

are in communication simultaneously.

However, use of full duplex/echo is not universal, and all

terminal emulators allow you to switch on and off the facility. If,

for example, you are talking into a half-duplex system (i.e. no

echo), your screen would appear totally blank. In these

circumstances, it is best if your software reproduces on the screen

your keystrokes.

** Page 20

However, if you have your computer set for half-duplex and the host

computer is actually operating in full duplex. each letter will

appear twice--once from the keyboard and once, echoing from the host,

ggiiwiinngg tthhiiss ssoorrtt ooff eeffffeecctt. Your terminal

emulator needs to able to toggle between the two states.

Data Format/Parity Setting - In a typical asynchronous protocol, each

character is surrounded by bits to show when it starts, when it ends,

and to signify whether a checksum performed on its binary equivalent

comes out even or odd. The character itself is described, typically,

in 7 bits and the other bits, start, stop and parity, bringing the

number up to 10. (See chapter 2.) However, this is merely one very

common form, and many systems use subtle variants -- the ideal

terminal emulator software will let you try out these variants while

you are still on line. Typical variants should include:

Word length Parity No stop bits

7 Even 2

7 Odd 2

7 Even 1

7 Odd 1

8 None 2

8 None 1

8 Even 1

8 Odd 1

(NB although the ASCII character set is 7 bit, 8 bits are sometimes

transmitted with a ~padding~ bit; machine code instructions for 8-bit

and 16-bit machines obviously need 8-bit transmissions.)

Show Control Characters - This is a software switch to display

characters not normally part of the text that is meant to be read but

which nevertheless are sent by the host computer to carry out display

functions, operate protocols, etc. With the switch on, you will see

line feeds displayed as ^J, a back-space as ^H and so on; see

Appendix IV for the usual equivalents.

Using this device properly you will be able, if you are unable to

get the text stream to display properly on your screen, to work out

what exactly is being sent from the host, and modify your local

software accordingly.

** Page 21

Control-Show is also useful for spotting 'funnies' in passwords and

log-on procedures--a common trick is to include ^H (backspace) in the

middle of a log-on so that part of the full password is overwritten.

(For normal reading of text, you have Control-Show switched off, as

it makes normal reading difficult.)

Macros - This is the US term, now rapidly being adopted in the UK,

for the preformatting of a log-on procedure, passwords etc. Typical

connecting procedures to US services like The Source, CompuServe, Dow

Jones etc are relatively complicated, compared with using a local

hobbyist bulletin board or calling up Prestel. Typically, the user

must first connect to a packet- switched service like Telenet or

Tymnet (the US commercial equivalents of BT's PSS), specify an

'address' for the host required (a long string of letters and

numbers) and then, when the desired service or 'host' is on line,

enter password(s) to be fully admitted. The password itself may be in

several parts.

The value of the 'macro' is that you can type all this junk in

once and then send off the entire stream any time you wish by means

of a simple command. Most terminal emulators that have this feature

allow you to preformat several such macros.

From the hacker's point of view, the best type of macro facility

is one that can be itself addressed and altered in software:

supposing you have only part of a password: write a little routine

which successively tries all the unknowns; you can then let the

computer attempt penetration automatically. (You'll have to read the

emulator's manual carefully to see if it has software-addressable

macros: the only people who need them are hackers, and, as we have

often observed, very few out-and-out hacker products exist!)

Auto-dial - Some modems contain programmable auto-diallers so that

frequently-called services can be dialled from a single keyboard

command.

Again the advantage to the hacker is obvious--a partly- known

telephone number can be located by writing some simple software

routine to test the variables.

However, not all auto-dial facilities are equally useful. Some

included in US-originated communications software and terminal

emulators are for specific 'smart' modems not available

elsewhere--and there is no way of altering the software to work with

other equipment. In general, each modem that contains an auto-dialler

has its own way of requiring instructions to be sent to it. If an

auto-dialling facility is important to you, check that your software

is configurable to your choice of auto-dial modem.

Another hazard is that certain auto-diallers only operate on the

multi-frequency tones method ('touch-tone') of dialling used in large

parts of the United States and only very slowly being introduced in

other countries. The system widely used in the UK is called 'pulse'

dialling. Touch-tone dialling is much more rapid than pulse dialling,

of course.

** Page 22

Finally, on the subject of US-originated software, some packages

will only accept phone numbers in the standard North American format

of: 3-digit area code, 3-digit local code, 4-digit subscriber code.

In the UK and Europe the phone number formats vary quite

considerably. Make sure that any auto-dial facility you use actually

operates on your phone system.

Format Screen - Most professional on-line and time-share services

assume an 80-column screen. The 'format screen' option in terminal

emulators may allow you to change the regular text display on your

micro to show 80 characters across by means of a graphics 'fiddle';

alternatively, it may give you a more readable display of the stream

from the host by forcing line feeds at convenient intervals, just

before the stream reaches the right- hand margin of the micro's

'natural' screen width.

Related to this are settings to handle the presentation of the

cursor and to determine cursor movement about the screen-- normally

you won't need to use these facilities, but they may help you when

on-line to some odd-ball, non-standard service. Certain specific

'dumb' terminals like the VT52 (which has become something of a

mainframe industry standard) use special sequences to move the cursor

about the screen--useful when the operator is filling in standard

forms of information.

Other settings within this category may allow you to view

characters on your screen which are not part of the normal character

set. The early Apples, for example, lacked lower case, presenting

everything in capitals (as does the ZX81), so various ingenious

'fixes' were needed to cope. Even quite advanced home computers may

lack some of the full ASCII character set, such oddities as the tilde

~ or backslash \ or curly bracket { }, for example.

Re-assign - keyboard A related problem is that home micro keyboards

may not be able to generate all the required characters the remote

service wishes to see. The normal way to generate an ASCII character

not available from the keyboard is from Basic, by using a Print

CHR$(n) type command. This may not be possible when on-line to a

remote computer, where everything is needed in immediate mode. Hence

the requirement for a software facility to re-assign any little-used

key to send the desired 'missing' feature. Typical requirements are

BREAK~ ESC, RETURN (when part of a string as opposed to being the end

of a command) etc. When re-assigning a series of keys, you must make

sure you don't interfere with the essential functioning of the

terminal emulator.

** Page 23

For example, if you designate the sequence ctrl-S to mean 'send a DC1

character to the host', the chances are you will stop the host from

sending anything to you, because ctrl-S is a common command (some-

times called XOF) to call for a pause--incidentally, you can end the

pause by hitting ctrl-Q. Appendix IV gives a list of the full ASCII

implementation and the usual 'special' codes as they apply to

computer-to-computer communications.

File Protocols - When computers are sending large files to each

other, a further layer of protocol, beyond that defining individual

letters, is necessary. For example, if your computer is automatically

saving to disk at regular intervals as the buffer fills up, it is

necessary to be able to tell the host to stop sending for a period,

until the save is complete. On older time-share services, where the

typical terminal is a teletypewriter, the terminal is in constant

danger of being unable mechanically to keep up with the host

computer's output. For this reason, many host computers use one of

two well-known protocols which require the regular exchange of

special control characters for host and user to tell each other all

is well. The two protocols are:

Stop/Start - The receiving computer can at any time send to the host

a Stop (ctrl-S) signal, followed by, when it is ready a Start,

(ctrl-Q).

EOB/ACK - The sending computer divides its file into a blocks (of any

convenient length); after each block is sent, an EOB (End of Block)

character is sent (see ASCII table, Appendix IV). The user's computer

must then respond with a ACK (Acknowledge) character.

These protocols can be used individually, together or not at all.

You may be able to use the 'Show Control Codes' option to check

whether either of the protocols are in use. Alternatively, if you

have hooked on to a service which for no apparent reason, seems to

stop in its tracks, you could try ending an ACK or Start (ctrl-F or

ctrl-S) and see if you can get things moving.

File transmission - All terminal emulators assume you will want to

send, as well as receive, text files. Thus, in addition to the

protocol settings already mentioned, there may be additional ones for

that purpose, e.g. the XMODEM protocol very popular on bulletin

boards. Hackers, of course, usually don't want to place files on

remote computers.....

Specific terminal emulation - Some software has pre-formatted sets of

characteristics to mimic popular commercial 'dumb' terminals. For

example, with a ROM costing under £60 fitted to a BBC micro, you can

obtain almost all of the features of DEC's VT100 terminal, which

until recently was regarded as something of an industry-standard and

costing just under £1000.

** Page 24

Other popular terminals are the VT52 and some Tektronix models, the

latter for graphics display. ANSI have produced a 'standard'

specification.

Baudot characters - The Baudot code, or International Telegraphic

Code No 2, is the 5-bit code used in telex and telegraphy -- and in

many wire-based news services. A few terminal emulators include it as

an option, and it is useful if you are attempting to hack such

services. Most software intended for use on radio link-ups (see

Chapter 10) operates primarily in Baudot, with ASCII as an option.

Viewdata emulation - This gives you the full, or almost full,

graphics and text characters of UK-standard viewdata. Viewdata tv

sets and adapters use a special character-generator chip and a few,

mostly British-manufactured, micros use that chip also-- the Acorn

Atom was one example. The BBC has a teletext mode which adopts the

same display. But for most micros, viewdata emulation is a matter of

using hi-res graphics to mimic the qualities of the real thing, or to

strip out most of the graphics. Viewdata works on a screen 40

characters by 24 rows, and as some popular home micros have 'native'

displays smaller than that, some considerable fiddling is necessary

to get them to handle viewdata at all.

In some emulators, the option is referred to as Prestel or

Micronet--they are all the same thing. Micronet-type software usually

has additional facilities for fetching down telesoftware programs

(see Chapter 10).

Viewdata emulators must attend not only to the graphics

presentation, but also to split-speed operation: the usual speeds are

1200 receive from host, 75 transmit to host. USA users of such

services may get them via a packet-switched network, in which case

they will receive it either at 1200/1200 full duplex or at 300/300.

Integrated terminal emulators offering both 'ordinary'

asynchronous emulation and viewdata emulation are rare: I have to use

completely different and non-compatible bits of software on my own

home set-up.

Modems

Every account of what a modem is and does begins with the classic

explanation of the derivation of the term: let this be no exception.

Modem is a contraction of modulator-demodulator.

A modem taking instructions from a computer (pin 2 on RS232C)

converts the binary 0's and 1's into specific single tones, according

to which 'standard' is being used. In RS232C/V24, binary 0 (ON)

appears as positive volts and binary 1 (OFF) appears as negative

volts.

** Page 25

The tones are then fed, either acoustically via the telephone

mouth-piece into the telephone line, or electrically, by generating

the electrical equivalent direct onto the line. This is the

modulating process.

In the demodulating stage, the equipment sits on the phone line

listening for occurrences of pre-selected tones (again according to

whichever 'standard' is in operation) and, when it hears one,

delivers a binary 0 or binary 1 in the form of positive or negative

voltage pulses into pin 3 of the computer's serial port.

This explanation holds true for modems operating at up to 1200

baud; above this speed, the modem must be able to originate tones,

and detect them according to phase as well, but since higher-speed

working is unusual in dial-up ports--the hacker's special interest,

we can leave this matter to one side.

The modem is a relatively simple bit of kit: on the transmit side

it consists of a series of oscillators acting as tone generators, and

on receive has a series of narrow band-pass filters. Designers of

modems must ensure that unwanted tones do not leak into the telephone

line (exchanges and amplifiers used by telephone companies are

sometimes remotely controlled by the injection of specific tones) and

also that, on the receive side, only the distinct tones used for

communications are 'interpreted' into binary 0s or 1s. The other

engineering requirements are that unwanted electrical currents do not

wander down the telephone cable (to the possible risk of phone

company employees) or back into the user's computer.

Until relatively recently, the only UK source of low-speed modems

was British Telecom. The situation is much easier now, but

de-regulation of 'telephone line attachments', which include modems,

is still so recent that the ordinary customer can easily become

confused. Moreover, modems offering exactly the same service can vary

in price by over 300%. Strictly speaking, all modems connected to

the phone line should be officially approved by BT or other

appropriate regulatory authority.

At 300 baud, you have the option of using direct-connect modems

which are hard-wired into the telephone line, an easy enough

exercise, or using an acoustic coupler in which you place the

telephone hand-set. Acoustic couplers are inherently prone to

interference from room-noise, but are useful for quick lash-ups and

portable operation. Many acoustic couplers operate only in

'originate' mode, not in' answer'. Newer commercial direct- connect

modems are cheaper than acoustic couplers.

** Page 26

At higher speeds acoustic coupling is not recommended, though a

75/1200 acoustic coupler produced in association with the Prestel

Micronet service is not too bad, and is now exchanged on the

second-hand market very cheaply indeed.

I prefer modems that have proper status lights--power on, line

seized, transmit and receive indicators. Hackers need to know what is

going on more than most users.

The table below shows all but two of the types of service you are

likely to come across; V-designators are the world-wide 'official'

names given by the CCITT; Bell-designators are the US names:

Service Speed Duplex Transmit Receive Answer

Designator 0 1 0 1

V21 orig 300(*) full 1180 980 1850 1650 -

V21 ans 300(*) full 1850 1650 1180 980 2100

V23 (1) 600 half 1700 1300 1700 1300 2100

V23 (2) 1200 f/h(**) 2100 1300 2100 1300 2100

V23 back 75 f/h(**) 450 390 450 390 -

Bell 103 orig 300(*) full 1070 1270 2025 2225 -

Bell 103 ans 300(*) full 2025 2225 1070 1270 2225

Bell 202 1200 half 2200 1200 2200 1200 2025

(*)any speed up to 300 baud, can also include 75 and 110 baud

services

(**)service can either be half-duplex at 1200 baud or asymmetrical

full duplex, with 75 baud originate and 1200 baud receive (commonly

used as viewdata user) or 1200 transmit and 75 receive (viewdata

host)

The two exceptions are:

V22 1200 baud full duplex, two wire

Bell 212A The US equivalent

These services use phase modulation as well as tone.

British Telecom markets the UK services under the name of

Datel--details are given in Appendix V.

BT's methods of connecting modems to the line are either to

hard-wire the junction box (the two outer-wires are the ones you

usually need)--a 4-ring plug and associated socket (type 95A) for

most modems, a 5-ring plug and associated socket (type 96A) for

Prestel applications (note that the fifth ring isn't used)--and, for

all new equipment, a modular jack called type 600. The US also has a

modular jack, but of course it is not compatible.

** Page 27

Modern modem design is greatly aided by a wonder chip called the

AMD 7910. This contains nearly all the facilities to modulate and

demodulate the tones associated with the popular speed services, both

in the CCITT and Bell standards. The only omission--not always made

clear in the advertisements--are services using 1200/1200

full-duplex, ie V22 and Bell 212A.

Building a modem is now largely a question of adding a few

peripheral components, some switches and indicator lights, and a box.

In deciding which 'world standard' modem to purchase, hackers should

consider the following features:

Status lights you need to be able to see what is happening on the

line.

Hardware/software switching - cheaper versions merely give you a

switch on the front enabling you to change speeds, originate or

answer mode and CClTT or Bell tones. More expensive ones feature

firmware which allows your computer to send specially formatted

instructions to change speed under program control. However, to make

full use of this facility, you may need to write (or modify) your

terminal emulator.

Auto-dial - a pulse dialler and associated firmware are included in

some more expensive models. You should ascertain whether the

auto-dialer operates on the telephone system you intend to hook the

modem up to--some of the US 'smart' modems present difficulties

outside the States. You will of course need software in your micro to

address the firmware in the modem --and the software has to be part

of your terminal emulator, otherwise you gain nothing in convenience.

However, with appropriate software, you can get your computer to try

a whole bank of numbers one after the other.

D25 connector - this is the official 'approved' RS232CN24 physical

connection--useful from the point-of-view of easy hook-up. A number

of lower-cost models substitute alternative DIN connectors. You must

be prepared to solder up your own cables to be sure of connecting up

properly.

Documentation I always prefer items to be accompanied by proper

instructions. Since hackers tend to want to use equipment in

unorthodox ways, they should look for good documentation too.

** Page 28

Finally, a word on build-your-own modems. A number of popular

electronics magazines and mail-order houses have offered modem

designs. Such modems are not likely to be approved for direct

connection to the public telephone network. However, most of them

work. If you are uncertain of your kit-constructing skills, though.

remember badly-built modems can be dangerous both to your computer

and to the telephone network.

Test Equipment

Various items of useful test equipment occasionally appear on the

second-hand market--via mail-order, in computer junk shops, in the

flea-market section of exhibitions and via computer clubs.

It's worth searching out a cable 'break-out' box. This lets you

restrap a RS232C cable without using a soldering iron--the various

lines are brought out on to an accessible matrix and you use small

connectors to make (or break) the links you require. It's useful if

you have an 'unknown' modem, or an unusually configured computer.

Related, but much more expensive, is a RS232C/V24 analyser --this

gives LED status lights for each of the important lines, so you can

see what is happening.

Lastly, if you are a very rich and enthusiastic hacker, you can

buy a protocol analyser. This is usually a portable device with a

VDU, full keyboard, and some very clever firmware which examines the

telephone line or RS232C port and carries out tests to see which of

several popular datacomms protocols is in use. Hewlett Packard do a

nice range. Protocol analysers will handle synchronous transmissions

as well as synchronous. Cost: £1500 and up...and up.