Acorn Computers Limited, 4a Market Hill, Cambridge CB2 3NJ. England. Telephone 0223 312772 SYSTEM 2 TECHNICAL MANUAL Chapter 1 - Introduction 2 - Parts List 3 - Electrical Assembly 4 - Mechanical Assembly 5 - Peripheral Devices 6 - Cassette Operating System Commands 7 - Operating System Control Characters 8 - Teletext Visual Display Unit 9 - O.S. Software Specification 10 - Circuit Diagrams & Descriptions 11 - System Expansion (c) COPYRIGHT ACORN COMPUTERS LTD 1980 ISSUE 1 JAN 1980 OCRed and converted to plain text by Ben Harris , April 1998. $Id: Sys2TM.txt,v 1.11 1998/05/21 20:25:18 bjh21 Exp $ Chapter 1 Introduction to system 2 The Acorn system 2 is a complete micro-computer available in kit form or fully assembled and tested. Using the popular Euro cards (100 x 160mm) an easily expanded system concept has been created and system 2 itself may be produced as an expansion of the two card system 1. The cards are housed in a 19" card frame with a backplane using DIN standard two part connectors. No peripheral devices are supplied with the basic system 2, it being left to the user to select the ASCII keyboard, CRT display, cassette tape recorder and power supplies best suited to the application. These parts are, of course, available with the system if required. The system may be furnished with professional connectors, front panels and a 19" case if desired. At the heart of system 2 is the Cassette Operating System (C.O.S.) software and the Acorn BASIC interpereter. This resident software provides powerful high level language facilities ideally suited for all applications. Chapters 2, 3 and 4 of this manual need to be referred to only if the system is in kit form. Chapters 9, 10 and 11 need only be referenced in case of system expansion. The Acorn BASIC is described in its separate manuals. 1-1 Chapter 2 - System 2 Parts List Unpack the kit carefully and ensure that no components are concealed within the packing materials. 6502 C.P.U. card PCB Printed Circuit Card 200,000 issue 3 or later IC1 6502 micro processor and 40 pin socket IC2 INS 8154 RAM/IO " IC3 2114 RAM and 18 pin socket IC4 2114 RAM " IC5 Not supplied IC6 Not supplied IC7 OS ROM and 24 pin socket IC8 INS 8154 RAM I/O not supplied but 40 pin socket is IC9 74LS20 and 14 pin socket IC1O 74LS139 and 16 pin socket IC11 74LS04 and 14 pin socket IC12 74LS00 and 14 pin socket XTAL 1MHz crystal R1-4 4 off 4K7 resistor Cl-2 47 or 100 nF capacitor C5-8 " C9 15 uF electrolytic capacitor NB C3, C4, C10, IC5, OC6, and IC13 are not fitted. IC8 is available as an accessory. 2-1 Cassette Interface PCB Printed Circuit Card 200,001 issue 2 or later IC1 CD4024B and 14 pin socket IC2 CD4024B " IC3 CD4011B " IC4 CD4013B " IC5 CD4001B " IC6 CD4024B " IC7 LM358 and 8 pin socket R1 3K9 resistor R2 47K resistor R3 4K7 resistor R4 1K resistor RS 10K resistor R6 4K7 resistor R7 4K7 resistor R8 47K resistor R9 4K7 resistor R10 4K7 resistor R11 10K resistor R12 47K resistor R13 4K7 resistor R14 470K resistor R15 4K7 resistor R16 4K7 resistor C1 100pF capacitor C2 1nF capacitor C3 10nF capacitor C4 1nF capacitor C5 15uF electrolytic capacitor C6 47 or 100 nF capacitor C7 Not fitted C8 22nF capacitor C9 47 or 100 nf capacitor C10 47 pr 100 nF capacitor 2-2 Visual Display Unit Interface PCB Acorn Computers Ltd. pt no 200,002 IC1 74LS139 decoder and 16 pin socket IC2 74LS02 NOR gate and 14 pin socket IC3 74LS13 Schmitt trigger and 14 pin socket IC4 MC6845 C.R.T. controller and 40 pin socket IC5, 6 & 7 74LS157 data selector and 16 pin socket IC8 & 9 2114-3 R.A.M. and 18 pin socket IC1O 74LS174 hex flip-flop and 16 pin socket IC11 INS 8208 buffer and 20 pin socket IC12 74LS86 excl. OR gate and 14 pin socket IC13 SAA5050 character generator and 28 pin socket IC14 74LS00 NAND gate and 14 pin socket Q1 BC197 transistor D1 1N4148 diode R1 1K resistor R2 1K resistor R3 470 R preset potentiometer R4 10K resistor R5 4K7 resistor not supplied R6 22K resistor R7 4K7 resistor R8 10K resistor R9-12 4 off 10K resistors R13 4K7 resistor R14 10K resistor R15 470R resistor R16 82R resistor R17 18K resistor not supplied R18 10K resistor R19 10K resistor not supplied R20-22 3 off 10K resistors Cl 10 or 15 uF electrolytic capacitor C2-7 6 off 47 nF capacitors C8 100 pF capacitor C9 150 pF capacitor NB For IC11 INS208 is equivalent to DP8304 2-3 Memory Board (issue 2) PCB Printed Circuit Card 200,003 issue 2 of later IC3-10 2114 RAM and 18 pin socket IC1, 2, 11-16 18 pin sockets supplied for extension RAM IC17 BASIC ROM and 24 pin socket IC18 24 pin socket supplied for extension ROM IC19 74LS42 and 16 pin socket IC20 74LS00 and 14 pin socket IC21 81LS95 or 97 and 20 pin socket IC22 INS8208 and 20 pin socket IC23 81LS95 or 97 and 20 pin socket IC24 74L42 and 16 pin socket IC25 74LS00 and 14 pin socket R1 1K resistor C1-7 7 off 47 or 100 nF capacitors C8 15uF electrolytic capacitor NB IC24 is a 74L42 from National Semiconductor only Also supplied for system 2 is 1 off Card Frame 173-12501L 4 off 64 way plugs 17-3704L 4 off 64 way sockets 17-3705F 8 off card guides 173-12553G 1 off Back-plane PCB - 200,011 Earthwire and solder tag 16 off 2.5 mm x 10 mm screws 8 off 2.5 mm nuts 2-4 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 R.C.A. 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. 3-1 The resistor colour code is shown here. [ Picture removed from ASCII version ] 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. 3-2 [ Picture removed from ASCII version ] Some of the integrated circuits used in the system employ M.O.S. 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 M.O.S., I.C.'s and the I.C.'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 I.C. is identified as shown:- [ Picture removed from ASCII version ] 3-3 The sockets will have either a 45 ???degree??? chamfer for pin 1 or a semi circular cut out as shown:- [ Picture removed from ASCII version ] 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 I.C. 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. [ Picture removed from ASCII version ] Do not crack the capacitor body when bending the leads. 3-4 The electrolytic capacitors are polarised and the positive end marked + must be located as indicated on the circuit cards. The crystal on the C.P.U. card is fitted as shown:- [ Picture removed from ASCII version ] 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:- [ Picture removed from ASCII version ] 3-5 Identify the I.C. 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 I.C. fits easily to the socket. [ Picture removed from ASCII version ] Take care that no I.C. pins get bent under the I.C. when inserting and remember that I.C.1 on the C.P.U. card is the opposite way round to its neighbours. 6502 C.P.U. 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 I.C.'s 5 and 6 together with their 16 pin sockets and the 5 volt regulator, I.C. 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 I.C.13 as marked on the silk screen. The INS 8154 RAM/IO chip is fitted at I.C. 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. I.C. 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. 3-6 [ Picture removed from ASCII version ] The operating system ROM is fitted in I.C. 7 and to use it the correct address selection links are required:- [ Picture removed from ASCII version ] To produce this a track must be broken on side 2 of the card as shown:- [ Picture removed from ASCII version ] 3-7 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 I.C. 7. In system 2 three connections from I.C. 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:- I.C. 2 pin 16 to backplane plug pin B 19 for CASIN I.C. 2 pin 17 to backplane plug pin B 20 for CASOUT I.C. 2 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. I.C. 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 I.C. 6 pin 3 to pin B 14 of the connector for the Not High Tone signal. 3-8 [ Picture removed from ASCII version ] V.D.U. Interface Card R5 is not required on this card. [ Picture removed from ASCII version ] 3-9 Memory Card This card carries the 4K of user RAM in I.C.'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 I.C. 17 and the high one in I.C. 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 I.C. 17 and again a link must be fitted in position ROM 6. [ Picture removed from ASCII version ] If desired a write protect switch for the RAM may be fitted by breaking the track marked between I.C.'s 15 and 21 and fitting a SPST switch across it. 3-10 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: [ Picture removed from ASCII version ] 3-11 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 C.P.U. 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 £5.00. The connections are:- C.P.U. 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. 3-12 Chapter 4 - Mechanical Assembly The Eurocard frame kit contains two end plates, two end plate front angles, two front tie bars with tapped strips, two rear connector mounting rails and eight fixing screws. Assemble the frame as shown:- [ Picture removed from ASCII version ] Note that viewing the frame from the front the 83rd guide position is on the right. 4-1 The standard allocation of the front of the cardframe is as shown:- [ Picture removed from ASCII version ] The 2.8" space is for a 5/12 volt power supply. Seven Eurocards spaced at 1" are allowed for with another one occupying part of the 7" space for a floppy disc controller. The left hand Eurocard location is reserved for the C.P.U. card when bus buffers are fitted to the backplane. The four card system 2 does not require buffers and so the cards are fitted as follows: 2.8" Space (power supply) 1" Space 1" C.P.U. 1" cassette interface 1" V.D.U. interface 1" memory board 1" Space 1" Space 7" Space (Floppy disc controller) Take the backplane board with its four connectors and lay it so that the board is between the upper and lower rear tie bars with the connector mounting ears on the front of the tie bars. Pin 1 of the connectors should be at the top i.e. the bus buffer I.C. locations are to the left hand side. Using eight of the 2.5 mm screws provided fix the connectors into the tapped holes on the rear tie bars that are numbered 21, 26, 31 and 36. Check that the power bus lines on the backplane are not shorted to the tie bars. 4-2 [ Picture removed from ASCII version ] With the backplane mounted in the frame take care that the protruding connector pins are not damaged in any way. Card and module guides may be fitted at multiples of 0.2" spacings by either clipping them into the holes, or onto the metal between the holes, of the tie bars:- [ Picture removed from ASCII version ] 4-3 Note the guides are fitted with the flat side to the left and the stepped side to the right. To fit the guides place them in the appropriate location on the rear tie bar and then push them onto the front tie bar where they snap into position. [ Picture removed from ASCII version ] To remove the guides push them to the rear whilst unclipping them from the front tie bar. Top and bottom pairs of guides are fitted into positions 20, 25, 30 and 35. It should now be possible to slide the four boards into the guides with their components facing to the right. The connectors should mate together but it may be necessary to loosen the screws securing the backplane connectors and move the backplane fractionally to allow the connectors to mate freely. With a fully expanded system card guides are fitted in positions 15, 20, 25, 30, 35, 40 and 45 with the slimmer module guides in positions 2, 12, 51 and 82. The Eurocard frame may be fitted in a 19" case giving protection for the printed circuit cards. Two front handles supplied with the case are positioned behind the card frame angles and four screws with plastic washers secure the frame and handles into the case. 4-4 One inch front panels may be fitted to the Eurocards, these are available with holes already cut for suitable front connectors for each board. The memory board only requires a blank panel. The front panel is fixed to the board using two plastic brackets also supplied as shown:- [ Picture removed from ASCII version ] 4-5 Chapter 5 - Peripheral Devices Power Supply System 2 requires a 5 volt stabilised supply at a current rating of not less than 1.5 amps (2 amps with 8K of RAM). This supply may be connected with two pieces of wire to the holes provided on the backplane (see page 3-11). If the 0 volt line of the supply is not earthed a safety earth should be connected to the card frame metalwork which is in turn connected to the earth connection on the backplane. If available a 5 volt - 3 amp module may be fitted in the left of the rack using four module guides and a 32 way socket on the left of the backplane. Keyboard A parallel ASCII keyboard is required to be fitted on the front of C.P.U. card. A 5 volt supply for the keyboard is available and the board requires a 'low for key depressed' strobe signal. The connections may be soldered to the front of the board as follows:- (top) 20 + 5 volts 19 Reset 18 Key strobe 17 Data bit 6 16 Data bit 5 15 Data bit 4 14 Data bit 3 13 Data bit 2 12 Data bit 1 11 Data bit 0 10 - 9 - 8 - 7 - 6 - 5 - 4 - 3 - 2 - (bottom) 1 0 volts 5-1 The reset is provided by a simple push button connected to 0 volts. This is often available as an extra key on ASCII keyboards. If desired a 20 way pcb header to the C.P.U. card in which case the keyboard connections are as follows:- 20 Data bit 5 19 Data bit 6 18 Data bit 4 17 Key strobe 16 Data bit 3 15 Reset 14 Data bit 2 13 + 5 volts 12 Data bit 1 11 0 volts 10 Data bit 0 9 - 8 - 7 - 6 - 5 - 4 - 3 - 2 - 1 - When connecting ribbon cable sockets to the headers ensure that the pin 1 identification arrows are together. Cassette Recorder System 2 uses a standard Audio Cassette Recorder for program and data storage. On the cassette interface card is a Computer Users Tape Standard interface which connects to the recorder as shown:- [ Picture removed from ASCII version ] 5-2 This recorder output consists of one of two tones, 2.4 KHz represents a logic 1 and 1.2 KHz a logic 0. Each bit i.e. 0 or 1 lasts for 3.3 mS giving an operspeed of 300 bits/ second. Both recording and playback are crystal controlled giving a low error rate and except on very cheap recorders whose speed may vary, no trouble should be experienced in transferring tapes from one machine to another. We recommend the use of a recorder with an input and output dedicated to external connection to an amplifier as opposed to the microphone and external loudspeaker connections on the cheaper recorders. These types usually have a 3 or 5 pin circular DIN connector which is wired as shown above. The input and output recorder levels as supplied are both 300 mV r.m.s. The output may be adjusted by changing R3 and input by changing R12 on the keyboard. The output should be adjusted so that the tones play back at a comfortable audio volume without excessive distortion. The input is then adjusted for reliable operation or using an oscilloscope, to give a waveform at pin 7 of I.C. 7 which is in excess of 2 volts peak to peak but is not being clipped. Visual Display Unit The V.D.U. interface drives a raster scanned cathode ray tube display device e.g. a monitor or a television. The format is 40 characters on 25 lines with teletext colour graphics. The standard output is a 1 volt/75 ohm composite video signal available on pin 15 on the front of the V.D.U. card with a 0 volt return on pin 14. A 75 ohm coaxial cable connected to this will drive a monitor directly. Also this signal may be used to drive a UHF modulator as shown below giving an interface to a standard television. 5-3 [ Picture removed from ASCII version ] Using the composite video output the different colours available from the VDU card will appear as different shades of grey, even on a colour monitor or television. To obtain a colour display the Red, Green and Blue drive signals from the card together with the required sychronising signals must be fed to either a T.T.L. level input colour monitor or a colour encoder/UHF modulator system. Note that if a BNC connector is mounted on an optional front panel for the video output the body of the connector i.e. 0v should be insulated from the panel. After switching on and entering the COS R3 on the V.D.U. card is adjusted to give stable characters of the correct width on the screen. 5-4