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 CPU 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:

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 CPU 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 ]

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 speed 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 IC 7 which is in excess of 2 volts peak to peak but is not being clipped.

Visual Display Unit

The VDU 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 VDU 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.

[ 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 TTL 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 VDU card is adjusted to give stable characters of the correct width on the screen.