Chapter 3 - Electrical Assembly

Assembly Technique

Four Eurocards and a backplane board have to be assembled for system 2, the components for each card are packed separately in plastic bags or on conducting foam.

Before attempting to assemble each card check that all the component parts are present and that none have been damaged. It is worthwhile taking a few minutes to make sure that all the components can be identified. Sometimes components will be substituted in case of supply difficulties. For instance, 0.047 uF capacitors may replace 0.1 uF capacitors shown on the parts list. The components substituted will in no way be detrimental to the system's operation. Also some manufacturers have similar but different type numbers e.g. for the CMOS a CD4011 from RCA may be replaced by an MM5611 from National Semiconductor.

For capacitors note that the value may be expressed in one of two ways:

100	nF	=	0.1	uF
10	nF	=	0.01	uF
 1	nF	=	1000	pF
0.1	nF	=	100	pF	etc.

Capacitors supplied with the Acorn cards are usually identified by a 3 digit number, the first two digits being the first two digits of the value and the third being the number of following zeros eg.

101	=	10 and one zero ie. 100	pF
103	=	10 and three zeroes ie. 10000 pf = 10 nF
473	=	47 and three zeros i.e. 47000 pF = 47 nF = 0.047 uF.

If in doubt about the capacitor values, count the number of each of type supplied in the kit and then identify them using the parts list quantities.

The resistor colour code is shown here.

The first and second bands give the resistor value and the decade band shows the number of zeros following:-

0	Black
1	Brown
2	Red
3	Orange	e.g.  Yellow, Violet, Orange
4	Yellow	is Yellow, Violet = 4,7 and
5	Green	Orange = 3 zeros i.e. 000.
6	Blue	So the value is 47000 ohms,
7	Violet	i.e.  47 kilo-ohms or 47K.
8	Grey
9	White

The tolerance band is red for +/- 2%, gold for +/- 5% or silver for +/- 10%.

Assembling the cards will require a considerable amount of soldering and a small electric soldering iron is essential with a diameter at the end of the bit not exceeding 0.1 inches. The iron should be rated between 10 and 30 watts and fine 22 gauge flux cored solder should be used. If you have never soldered before we advise you not try to assemble the cards without assistance as Acorn Computer Ltd. cannot accept responsibility for kits which have been improperly assembled. When soldering make sure 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 system 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 soldering on a board containing MOS IC's and the IC's should be kept on the conducting foam on which they are supplied until required.

The Acorn Printed Circuit cards are double sided, through hole plated glass fibre and are manufactured to the highest standards. A layer of green solder resist ensures that accidental solder splashes do not stick to the tracks and a clearly marked white silk screen indicates component positions. Examine the cards for faults or damage before proceeding. It is not necessary to solder through holes which connect one side of a board to the other and do not have a component lead in them and attempting to do so can break the through hole plating and thus the connection. All soldering should only be done on the opposite side of the board to the components (i.e. side 1).

The cards are each supplied with a full set of integrated circuit sockets. The sockets must be fitted the right way round, on the circuit board viewing it from the top. Pin 1 of an IC is identified as shown:-

The sockets will have either a 45° chamfer for pin 1 or a semi circular cut out as shown:

Note that on the Central Processor Unit IC1 is the opposite way round to the other sockets nearby. Fit the sockets one at a time and ensure that they are pressed fully down with no leads bent under the socket before first soldering two diagonally opposite pins at the corners. Check that the socket is the right way round and successfully fitted before soldering the rest of the pins.

There is no need to snip off the excess of the socket pins.

After the IC sockets the resistors and capacitors are fitted to the circuit cards. Identify the component from the component list and fit it to the board. Some capacitors will need to be fitted as shown.

Do not crack the capacitor body when bending the leads.

The electrolytic capacitors are polarised and the positive end marked + must be located as indicated on the circuit cards.

The crystal on the CPU card is fitted as shown:-

Again bend the leads away from the component body and lay the Crystal down on the board before soldering. Snip off any excess leads under the board.

The connector is fixed to a card using two 2.5 mm screws and nuts before soldering the pins. When all of the components are soldered the integrated circuits may be fitted in their sockets, pin 1 is identified by either a semicircle or a dot as shown:

Identify the IC type from the components list and plug it into the appropriate socket. If the leads are splayed out press them all in together as shown until the IC fits easily to the socket.

Take care that no IC pins get bent under the IC when inserting and remember that IC1 on the CPU card is the opposite way round to its neighbours.

6502 CPU

Printed circuit cards at issue 3 or later are supplied with complete systems. The NMI, IRQ and RST switches are not required. Also not supplied or required are IC's 5 and 6 together with their 16 pin sockets and the 5 volt regulator, IC 13. The electrolytic decoupling capacitor is fitted at C8 and C10 must not be used. This allows the Block zero signal to be connected to the backplane by fitting a wire link to the right hand side of IC13 as marked on the silk screen.

The INS 8154 RAM/IO chip is fitted at IC 2 and the ASCII keyboard connects to port B of it. A 20 way right angled header may be fitted for connecting the keyboard or wires can be soldered directly to the edge fingers. IC 8 is not used in the system but a second INS 8154 can be fitted here giving 16 I/O lines for user definition and so a socket for it is supplied.

The operating system ROM is fitted in IC 7 and to use it the correct address selection links are required:-

To produce this a track must be broken on side 2 of the card as shown:

and a wire link must be fitted from pin 5 to pin 9 of the ADR SEL location. The ROM IC marked COS (Cassette Operating System) can now be fitted in IC7.

In system 2 three connections from IC 2 to the backplane are required in order to connect to the cassette interface. On issue 3 boards these must be made with three pieces of fine insulated wire and they are as follows:

IC2 pin 16 to backplane plug pin B 19 for CASIN
IC2 pin 17 to backplane plug pin B 20 for CASOUT
IC2 pin 18 to backplane plug pin B 21 for NHITON

Cassette Interface

Printed Circuit Cards at issue 2 or later are supplied with complete systems. IC 8, the 8 digit display and the small keypad are not required for system 2.

Before fitting the 64 way plug the following pins MUST be cut off the plug as close to its body as possible. The pin numbers are marked on the body of the plug:-

A2 - 31		inclusive
Bl
B21 - 31	inclusive

A wire link is required from IC 6 pin 3 to pin B 14 of the connector for the Not High Tone signal.

VDU Interface Card

R5 is not required on this card.

Memory Card

This card carries the 4K of user RAM in IC's 3, 4, 5, 6, 7, 8, 9 and 10 and to address it a link must be fitted in position RAM 1. A further 4K of RAM (8 x 2114's) may be added at anytime and the extra sockets are supplied as standard.

If an issue 1 memory card is supplied the BASIC will be in a pair of EPROM's. The low one is fitted in IC 17 and the high one in IC 18, a link must then be fitted in position ROM 6.

Eventually all systems will be supplied with issue 2 memory cards and the BASIC in a single ROM. This ROM is fitted in IC 17 and again a link must be fitted in position ROM 6.

If desired a write protect switch for the RAM may be fitted by breaking the track marked between IC's 15 and 21 and fitting a SPST switch across it.

Back Plane

The backplane has provision for eight 64 way Euro card sockets and two 32 way power supply sockets. Side A is connected from pin to pin right across the back plane and carries the computer data bus, address bus, control lines, clock and power. Side B is left unconnected and may be wire wrapped to configure a user defined system of additional boards, extra I/O, interrupt priorities etc.

The left hand 64 way card location has its 16 bit address bus isolated from the other cards, and so it may not be used for system 2. System 2 connectors are fitted as shown:

Ensure that the connectors have pin 1 at the top and push their pins through the backplane. With each connector pushed hard against the backplane solder pins A1 and A32, check that the connectors are all lying flat on the back plane before soldering the rest of side A. The side B pins should not be soldered at all.

Four connections are necessary to connect the 6502 CPU card to the cassette interface. These may be made with soldered links but preferably are wire-wrapped. Simple hand wire-wrapping tools are available for about £5. The connections are:

CPU	card pin A29 to cassette interface pin B11, Ø2 clock
     "		 B19	   "	      "	       B12, CASIN
     "		 B20	   "	      "	       B13, CASOUT
     "		 B21       "	      "	       B14, NHITON

Finally a wire link is soldered on the left of the back plane to connect the system 0volts to the earthing wire. The earthing wire is soldered into the backplane and using a solder tag it is connected to the card frame metal work by the top left tie bar screw after the backplane has been mounted in the frame.

The backplane is now ready for use in system 2.