Atom Technical Manual



Carefully unpack the ATOM kit and check that all the component parts are present. ard that none are damaged. In case of supply difficulties substitute components may be supplied; for instance 100nF capacitors may replace 47nF parts.

For capacitors note that values may be expressed in two ways:

 O.1uF = 100nF
10nF = 10000pF
1nF = 1000pF
O.1nF = 100pF etc.

Capacitors supplied with Acorn products are usually identified by a three digit number, the first two digits being the first two digits of the value followed by a third digit giving the number of zeros. For example:

101 is 10 and one zero which is 100pF.
103 is 10 and three zeros which is 10000pF or 10nF.
473 is 47 and three zeros which is 47nF or 0.047uF.

Resistors are identified by coloured bands of which three at one end give the value with a fourth giving the components tolerance:
===== __

__ =====
    1   2   3   4    
The first and second bands give the first two digits of the value with the third indicating the number of following zeros:

For example consider a resistor with the coloured bands Yellow, Violet, Orange, space, Silver. This is 47 followed by three zeros, that is 47000 or 47K ohms. When writing the value of a resistor note that 4.7 Kohms is usually written 4K7, and 270 ohms as 270R.

__ Black is 0
  Brown is 1
  Red is 2
  Orange is 3
  Yellow is 4
  Green is 5
  Blue is 6
  Violet is 7
  Grey is 8
  White is 9
The fourth band identifies the tolerance as follows:
Any of these may be used with the ATOM.
__ Red is +or-2%
  Gold is +or-5%
  Silver is +or-10%

Solder jointAssembling the ATOM will require a considerable amount of soldering and a small electric soldering iron is essential. The iron should be rated between 10 and 30 Watts with a diameter at the end of the bit not exceeding 0.1 inches. Fine 22-gauge flux cored solder should be used. People with no previous experience of this type of work should not attempt the ATOM assembly without assistance. When soldering make sure that the component is well pushed on to the board as shown, use a minimum of solder, and once the solder has run remove the iron.

Some of the integrated circuits used in the ATOM employ MOS technology and they can be damaged by static electricity. As a general rule if there is no noticeable static charge in the area and no nylon clothes or carpets are present all will be well. An earthed soldering iron should be used when working on a board carrying MOS circuits.

The ATOM printed circuit board is double sided, through-hole plated glass fibre. A layer of green solder resist ensures that accidental solder splashes do not short out tracks and a white component legend clearly indicates where each part goes.

Examine the board for faults or damage before commencing work. Do not solder through holes on the board connecting tracks from one side to the other without a component in them as they may be damaged; in any case these holes should have solder resist over them.


Parts supplied with the basic ATOM kit are as follow

IC6 74LS138 TTL Decoder.
IC7 74LS02 TTL NOR Gate.
IC8 74LS00 TTL NAND Gate.
IC9 74LS04 TTL Inverter
IC20 MM52164 Read Only Memory.
IC22 6502-1MHz. Micro-Processor.
IC23 74LS138 TTL Decoder.
IC25 INS8255 Peripheral Interface.
IC26 7445 TTL Decoder/Driver.
IC27 81LS95 TTL Buffer.
IC28 81LS95 TTL Buffer.
IC29 DP8304 TTL Transceiver.
IC30 74LS138 TTL Decoder.
IC31 6847 Video Display Generator.
IC42 2114 Random Access Memory.
IC43 2114 Random Access Memory.
IC44 74LS393 TTL Counter.
IC45 74LS04 TTL Inverter.
IC46 LM358 Operational Amplifier.
IC47 74LS163 TTL Counter.
IC48 74LS00 TTL NAND Gate.
IC49 74LS139 TTL Decoder.
IC51 2114 Random Access Memory.
IC52 2114 Random Access Memory.
IC53 LM340T-5 Voltage Regulator.
IC54 LM340T-5 Voltage Regulator.
R1 4K7   R26 470R
R2 4K7   R27 470R
R3 470R   R28 470R
R4 470R   R29 4K7
R5 470R   R30 1K
R6 75 or 82R   R31 4K7
R7 8K2   R32 4K7
R8 3K3   R33 47K
R9 1K   R34 47K
R10 270R   R35 470K
R11 3K9   R36 4K7
R12 470R   R37 10K
R13-16 4 off 4K7   R38 1K
R17-25 9 off 4K7   R39-41 3 off 4K7
C1 10nF   C8 22uF *
C2 22uF *   C9 22 or 47nF
C3 22uF *   C10 10nF
C4 22uF *   C11 10uF *
C5 22uF *   C12 22uF *
C6 10nF   C13 22uF *
C7 47 or 100nF   C14-28 15 off 47nF

All of these resistors may be 0.25 or 0.33 Watt components with a tolerance of 5 or 10%.

The electrolytic capacitors (marked *) may be replaced by any value in the range 10 to 47uF. They should be rated at 10 volts or greater.

Other parts supplied with the basic ATOM kit include:

 Injection Moulded Case.
Printed Circuit Board 202,000
Four short and two long self-tapping screws.
Self-adhesive key surround.
Four stick-on feet.
Key Assembly and fixing screws.
Xl 3.58 MHz crystal.
X2 4.00 MHz crystal.
Ll 1 mH choke.
SK1 UHF Modulator.
SK2 7 pin DIN socket.
SK3 Power socket.
Q1,2 2 NPN transistors type BC107 or 184,237,239
Loudspeaker,adhesive pad and wire.
Loudspeaker Baffle.
Nut and screw for Regulator.
Heat sink.

52 sockets for the complete set of Integrated Circuits as follows.

 1 off 8 pin
7 off 14 pin
6 off 16 pin
24 off 18 pin
7 off 20 pin
3 off 24 pin
4 off 40 pin

The remainder of the components required for the extensions are listed in the appropriate sections later on.


The ATOM printed circuit board is double sided with the sides being identified by the legends “SIDE 1” and “SIDE 2”. All the IC sockets, the connectors PL/SK 1 to 7 and the passive components are mounted on side 2 and they are therefore soldered on side 1. PL 8 and the keyboard are mounted on side 1 and soldered on side 2. The component layout diagram is a view of the circuit board from side 2 and it will assist in identifying components whose legend is obscured on the board itself.

Read through the following instructions and then assemble the ATOM step by step.


Several links may be fitted to the PCB to cater for different requirements. None of the links need to be fitted for a minimal-configuration ATOM powered by an ATOM mains adaptor.

Side 2 layout (click to enlarge) click to enlarge


The sockets supplied with the ATOM are fitted to the circuit board in the locations for the ICs provided. Sockets are mounted on side 2 of the board and they are then soldered on side 1. Fit the sockets the right way round with pin 1 in the position identified on the circuit board as shown:

Intergrated circuit sockets

Pin 1 of the socket is identified by a chamfer or cut out as shown. Fit the sockets one at a time and ensure that they are pressed fully down with no pins bent under them. First solder two diagonally opposite pins and check that the socket is correctly fitted before soldering the remainder of the pins. There is no need to cut off the excess of the pins on side 1 of the board.


The passive components, that is the resistors and capacitors, are fitted next. Each resistor and capacitor position is marked on side 2 of the circuit board with a legend ie R7 or C12 and a line showing the two holes through which the leads pass. Identify each component from the parts list and bend its leads as shown before mounting it on side 2 of the board with the leads poking through to side 1. Solder the leads on side 1 and cut off any excess. The electrolytic capacitors are polarised and the positive ends of them must be towards the + sign on the board. Some electrolytics have an arrow or a band round them indicatinq the negative end and this must point or be away from the + sign. The 1mH choke is fitted in position Ll.

Passive components


The ATOM may already have its keyboard fitted; if not then proceed as follows. Carefully remove the key assembly from its protective backing and lay it key-top side down with the leads sticking upwards. Check that all of the leads are straight and correct any that are bent over. Taking great care not to bend any of the leads position the circuit board over the key assembly such that the keys will be on side 1 of the board, that is the side opposite to the ICs and passives etc .Lower the circuit board over the key leads so that they poke through the holes in the circuit board and then move the two parts until the fixing screw holes are aligned. Ensure that ALL of the key leads poke through the circuit board. Then fix the keyboard to the PCB using six self-tappinq screws with insulating washers to ensure that they do not short out the PCB tracks. Solder each key lead on to the side of the large plated through-holes in the circuit board pushing the leads over to the side of the hole where necessary. Do NOT fill the holes with solder; when the solder cools it may contract and break the plated-through connection. Do not allow blobs of solder to fall through the holes into the keys.


The power jack socket and the D.I.N. socket for the tape recorder are mounted on side 2 of the board and soldered on side 1. The power socket, SK3, has two lugs which are soldered into the board and the third connection is made with a short piece of wire, such as an offcut from a resistor. SK2 is a 7 pin socket which is held by a spring clip through two holes near the front of the board and by the seven soldered leads.

Plugs PL4, PL5, PL6, PL7, and PL8 are not required on the minimum configuration ATOM.


Fix the loudspeaker to side 1 of the board to the right of the keyboard using a double-sided sticky pad. Connect it to the two marked holes beside C5 using twin-cored wire and press the baffle over the loudspeaker.


This is mounted on side 2 with the two metal securing lugs through the board and soldered on side 1. The +5 volt and input signal wires are soldered into the two holes indicated on the board beside the modulator.


Voltage RegulatorBoth regulators and a heatsink should be fitted to all ATOMs. The first regulator is fitted as shown:

Bend the leads taking care not to break them off as they enter the body of the device. Fit the second regulator in an identical manner, and then slide the heatsink between the regulators and the PCB. Screw down the regulatcrs with the nuts and screws supplied, taking care that they do not short out the adjacent tracks on the PCB, and then solder the leads on side 1 of the board. Cut off any excess lead.


The two crystals for the ATOM stand upright about a quarter of an inch above side 2 of the board. If necessary bend the leads so that they pass through the holes at positions X1 (3.58M) and X2 (4.00M). Take care not to break off the lead where it enters the crystal and check that the metal can does not short to any adjacent components.


Two transistors, Ql and Q2, are fitted taking care that they are oriented as on the circuit board legend.


These may now be fitted in their sockets. Pin 1 of each device is identified by either a semi-circle or a dot as shown:

IC pin 1

Devices which are liable to damage by static electricity will be supplied in anti-static packing and they should not be removed from this until required for insertion into their sockets. Identify each device from the parts list and plug it into the appropriate socket. If the pins on the IC are splayed out, press them all in together as shown until the device fits easily into the socket:

IC pin alignment

Take great care that no IC pins get bent under the IC when inserting and ensure that all of the ICs are correctly aligned with pin 1 in the right place. If available, more 2114s may be fitted in positions IC 10 to 19 and 32 to 41 as described in the extensions sections.


After checking that all soldering on side 1 is satisfactory the circuit board may be fitted in the case. If any extension kits are available they may be fitted as described in the relevant sections of this manual. Take the top of the case, that is the piece with the cut-out for the key-board, and lay it down with the pillars and screw holes facing upwards. The assembled circuit board should be placed over the case with the component side 2 upwards. Screw, but do not tighten, four self-tapping screws into the four corner mounting pillars in the case thus lightly holding the board in the case. Move the board in the case so that all the keys clear the edges of the cut-out in the top and then tighten up the four screws

The bottom of the case is next placed over the circuit board and moved so that it lines up with the top. If the connectors do not then appear centrally in the cut-outs on the rear of the case bottom some adjustment of the position of the board in the top of the case may be required. Finally put two long self-tapping screws into the holes in the case bottom and fix the bottom to the top by tightening them up.

Four self adhesive feet may be stuck to the bottom of the case and the surround for the keys is stuck to the top of case when its backing is peeled off revealing a layer of adhesive.


The ATOM screen display may be on a domestic television or a video monitor. To connect to a domestic television use the phono to co-axial connecting lead supplied. The phono plug goes into SK1 (the modulator output) and the other end of the lead goes to the aerial socket of the television. If the television has a graduated tuning scale, set it to channel 36.

A 1 volt into 75 ohm composite video signal for monitors is available on pin 9 of PL4 with a corresponding 0 volt return on pin 10. These are indicated on the following diagram.

Composite video out

The normal ATOM produces a monochrome picture, even if a colour television or monitor is used. The PAL encoder circuit board can be fitted inside the case to give a colour picture.


The basic ATOM requires a power source of 8 volts DC which is regulated on the circuit board by IC53 and IC54 to produce a stabilised 5 volts. The ATOM high-efficiency mains adaptor provides such a source, and it is fitted with a jack plug for connecting to the ATOM. The high-efficiency mains adapter is capable of delivering up to 1600 mA and powering a fully-expanded ATOM using the low-power memory that is currently supplied. This current is obtained as follows:

Minimal system 2K RAM 750 mA
10K expansion memory at 75 mA per K 750 mA
Bus buffers and 6522 VIA 100 mA

Note that ATOMs supplied before October 1980 used normal-power RAM memory devices which have a current consumption of 100 mA per K; the total current consumption of a fully-expanded ATOM usinq normal-power memory is therefore too great for the ATOM mains adapter.


The ATOM may alternatively be powered by a 5v regulated supply. In this case two links, LK6 and LK7, should be fitted to the board. The power distribution on the board is normally separated into two sections; one for each regulator. These sections should be joined together by fitting a link at LK6 on the circuit board. The regulators should be bypassed by fitting link LK7 on the board. The power socket, SK3, is now for a stabilised 5 volt input and the mains adaptor must NOT be plugged into it. This part should be marked clearly on a label attached near the power socket on the back of the ATOM.

A circuit far providing a stabilised 5 volts at up to 3000 mA capable of powering a fully expanded ATOM together with any Acorn eurocard is given here:

5 volt power supply Capacitor = 1500µF 16V.


When the T.V. or monitor is connected the power connector may be inserted into SK3. The ATOM is now on. Press the break key to reset the ATOM; the screen should now show:

>cursor block (black)

At this stage if a television is being used it should be tuned until a display is found. Both monitors and televisions may now require adjustment of the vertical hold control. After further fine tuning in the case of the television to obtain a sharp display, the brilliance control should be set so that the background is slightly grey rather than black. The contrast control is then adjusted to make the characters as bright as required.

If no display is found check that all the connections are correct and in the case of ATOMs assembled from kits check that the construction procedure has been followed correctly. when the display on the screen is correct refer to the book ‘Atomic Theory and Practice’ for details of how to program the ATOM.


Programs or data may be stored on standard audio tape using a domestic tape or cassette recorder. The ATOM contains a Computer Users Tape Standard (CUTS) interface, and connection to the recorder is by a 3 pin D.I.N. plug in SK2. Pin 1 is the output to the recorder, pin 2 is the 0 volt or common connection, and pin 3 is the input to the ATOM from the recorder. These are shown in the diagram:

atm_15_1.gif Back view of DIN plug
i.e. cable side

Only a 3 pin lead should be used as the other 4 pins of SK2 carry digital outputs which may damage recorders with 5 or 7 pin connectors.

Both recording and playback on the ATOM are crystal controlled and, except on very cheap recorders whose speed may vary, no trouble should be experienced when transferring tapes from one ATOM to another. The use of recorders with an input and output dedicated to connection to an external amplifier is recommended as opposed to those with only microphone and earpiece facilities; however good results should be obtained with most machines.

The setting up of the recorder using a test program is described on page 8 of the book ‘Atomic Theory and Practice’. Should correct results not be obtained the signal levels may require adjustment. Both the input and output are normally set to a level of 300 mV R.M.S. The output can be adjusted by changinq the value of R29 so that the recorder’s VU meter reads 70% on recording (20% = -3dB). The gain of the input amplifier on the ATOM may be changed by altering R34; hi-fi cassette decks are generally more sensitive and often require that this resistor is changed from 47K to 220K.


Pins 4,5,6 and 7 of SK2 carry bits 1,2,0 and 3 respectively of port C of the INS8255 (address B002). These are normally set to output mode and they will drive a standard TTL load. Bits 0 and 1 also drive the tape interface, bit 2 bleeps the loudspeaker, and bit 3 selects between the two possible sets of four colours. Refer to the book Atomic Theory and Practice‘ for further details.