INMC 80 News


October–December 1981, Issue 5

Page 30 of 71

N2 Keyboard

by David Pears


Nascom owners with keyboard problems (in my case a dry joint in one of the wire links) are confronted with a complete absence of information in the manual. Even a copy of the keyboard circuit diagram (with wrong pin number for PL3) from a friendly Nascom dealer only took me part of the way and the following notes may help others track down faults. The 56 keys (ignoring Reset and the duplication of SPACE) are at the intersections of 8 row conductors and 7 conductors, These are shown as column conductors and row conductors respectively on the keyboard circuit diagram, and also the Bits & P.C.s keypad circuit diagram but I am using Nas Sys terminology. I denote the rows R1 to R8 and the columns by their corresponding bits D0 to D6 in the data bus. The layout is then as in the following table which also gives pin numbers of keyboard ICS for the row interrogating pulses and pin numbers of PL3 for the columns.


* IC5 pins in brackets.

Basically the computer finds out which key is depressed by pulsing the row conductors in cyclic sequence and seeing which data line picks up pulses. For example “5” is detected as pulses on D2 at the time of pulsing R2. The start of the sequence is marked by making the pulses an R1 much wider than those on R2 to R8. IC6, IC3 and IC5 form a counter circuit driven by pulses on PL3/11 and 10 under software control, ie part of the Nas Sys keyboard routine, and the interrogating pulses appear on the pins of IC5 in the table above. The pulses picked up by the column conductors are buffered by IC9 and IC1 and latched by IC8, IC7, IC2 and half of IC4. The latches are cleared by a clear pulse from the counter circuit, (which also includes the other half of IC 4}.

Nas Sys forms a word identifying row and column when it detects anything on any of D0 to D6 at the right time and uses a table to calculate the ASCII code for the corresponding character. The correspondence can be completely arbitrary, since a table is used,

Page 30 of 71