Volume 1 · Number 1 · August 1981

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During the access the data to the V.D.U. is disabled by taking the enable lines to ICs 10 and 11 to +5 volts. A monostable, IC 16 can be used to lengthen the write pulse to reduce flashes on the screen.

The 2114 is a 1K x 4 bit static RAM, so a pair of 2114s needed for each 1K bytes. Identical address lines on each of the four RAM chips are tied together, and simlarly the eight data lines from the two pairs of chips are interconnected. When an address is accessed data is only interchanged with the pair of chips for which the CS signal is low, which is determined by the state of line 4 from the character generator socket for V.D.U. access, and by the state of A10 for processor access. The direction of the data transfer is determined by the WR signal.

The layout of the circuit is not particularly critical. The commonest problem is caused by coupling between the lines from pins 21 – 24 of the character generator socket. These lines carry signals derived from the crystal oscillator on the C.P.U. board via a divider chain. If a long ribbon cable is used to connect the unit to the main board interaction between these adjacent lines may cause screen Jitter. Fortunately the solution is simple – just separate the lines from the main cable.

Modifications To The Nascom

The Nascom character generator socket has two unconnected lines. These are used to connect bit 7 of the V.D.U. RAM to the unit and extra data output from the unit. This does not affect the operation of the Nascom if the unit is disconnected and the 6576 is replaced in its original socket. The following modifications should be made to the Nascom board (IC numbers prefixed by an N refer to the numbers used in the Nascom 1 manual ).

Pin 11 of N IC 17 should be bent horizontal so that it is not in contact with its socket and connected to N IC 15 pin 1. Similarly, pin 10 of N IC 15 is bent horizontal and connected to pin 10 of the character generator socket on the Nascom. Pin 12 of N IC 20 is connected to N IC 17 pin 18 and pin 19 of N IC 17 is conected to pin 14 of the character generator socket. The connections to N IC 15 are necessary because in the standard Nascom 1 V.D.U. the last bit in each line of the characters is set to zero; this leaves a gap between each character, which is not wanted in the case of graphics characters. Please note that pin 11 of N IC 17 and pin 10 of N IC 15 are the only pins which should be bent (this is easier than tracing tracks and then cutting the wrong one!).

The unit is designed specifically for a Nascom 1. It should not be difficult to adapt it for a Nascom 2, because you already have a socket decoded for a graphics ROM. I have not tried adapting the circuit because none of my friends with Nascom 2’s will let me perform the necessary surgery!

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