Micro­power

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

Construction

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