Volume 2, Number 3 – July 1982

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A6=1, A5=0, A4=0 and A7=0, giving access to the 16 registers from address 40H to 4FH (hex). The data-out, in BCD form, is present on the data lines D0-D5. R/W signals are directly connected to the control BUS of your Nascom.

IC 3 is only required by the “sporty” Nascoms running at 4 Mhz. (You mean all Nascoms don’t run at 4 Mhz??-Ed). It provides a Wait signal of approxiamately 1uS to allow the MM 58174 time to output data to the Data Bus. The DBDR signal is not needed by the Nascom 2’s and I am not too sure if it is in fact needed by all versions of Nascom 1’s. (see Nascom Doc)

As previously mentioned, 2 Ni-Cad batteries are used to keep power on during computer “days off”. These are kept charged via T1 and R1 when 5V is applied to the circuit. In “stand-by” made, the diode D1 ensures that the 2.2V powers the MM 58174. It is necessary, when 5V operations cease, that the /WR, pin 3, is kept at a sufficiently high potential to stop any random writing to the MM 58174 registers. R4 provides this potential and the 2 NOR gates, connected in series, provide the necessary isolation from the computers Control Bus. Finally, the variable capacitor VC1 allows accurate callibration.

List of Components

R115 KohmR21 KohmR34.7 Kohm
R41 KohmR5240 ohmR6500 ohm
R71 KohmR81 Kohm
C168 pFC2100uFVC150 pF
X1Quartz 32,768 Khz
IC 1MM 58174IC 274LS138IC 374121
IC 47433
Batt.2 rechargeable Ni-Cad batteries


I think anyone attempting to build the clock will already have sufficient skill to work out a simple Vero board layout. In view of the small number of components, it should not be difficult. Myself, I used a 10cm x 12cm wire wrap board. There was enough space for all the components and the Ni-Cad batteries housed in a plastic container (transistor radio type). As I have a “home built” case for my Nascom, I found no difficulty installing the board, wiring it directly into the Nascom Bus.

NasBus Connections

1Ground13/DBDR(N1s only)
23Wait (4Mhz only)26IORQ

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