80-Bus News


July–August 1984 · Volume 3 · Issue 4

Page 25 of 43

Pins A and B are the input and output of a sensing coil and C and D are the input and output of a driver coil which is pulsed by an output from a 74145 BCD – decimal decoder/​driver. In the unexpanded keyboard, the keys are arranged in a matrix of 8 driver lines (Dr0 – Dr7) and 6 sensing lines (S0 – S5) and one of the 48 points is not used. In the expanded version, an additional sense line is employed (S6) to give 56 points in the matrix, with the new Shift key added to give a total of 57. The sense outputs are processed to give data bits 0 – 5 on the CPU bus (or bits 0 – 6 in the expanded version). A browse through a circuit diagram of the keyboard (CK9009, April 1978) showed that half of IC1 (CA3086 transistor array) and half of IC3 (7400 quad 2-input NAND gate) are available. The buffering for the sense lines is provided by single transistors within each of the two 3086 arrays (IC1 and 9), and up to two new sense lines could be connected with the unused part of ICl. The circuit diagram also shows each buffered sense line and associated pull-up resistor connected to individual flip-flops each formed from 2 of the NAND gates in a 7400 (ICs 2,7 and 8). It is a simple matter to implement another flip-flop for bit 6 from the two spare gates available in IC3 and connect this to pin 7 of the keyboard socket. Fitting in the three resistors for the collector pull-up, emitter bias and base of the IC transistor buffer calls for some care.

The Nascom 1 keyboard circuit diagram shows a particular layout of characters against sense/​driver intersections in the key matrix but the actual order is slightly different (Murphy’s Law). For the issue B board, the following order was found: (issue A boards may be different)


The sense outputs are connected to IC9 and 1 as follows:

SenseICPinIC pin for output
to data bus
New sense line61911

The Driver lines are supplied from IC5 pins 1 – 7 and 9 (corresponding to Dr0 – Dr7). The new sense line could have been connected to ICl pin 6 but the circuit layout means that the connection to pin 9 was easier to arrange.

Board surgery can now commence! The holes for the new pins must be drilled accurately with a 1 mm bit and for most of the holes required, simply match up

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