Nascom News­let­ter

The RESET and CLOCK inputs of IC 1 are brought out to the host computer via SKT 1. The WRITE ENABLE (WE, active LOW) inputs of the memory chips are connected together and also go out to the Nascom via SKT 1 as do the 8 data lines and the ground connection. (Power may be obtained from the host if its supply is adequate, although the prototype had an independent supply. About 200mA at 5 volt is required.)

The eight data lines are connected directly to a PIO port on the Nascom without an intermediate buffer. Thsi simplifies the design but care must be taken when writing driver software: see later.

A buffer is definitely needed between the data lines and the target system. This function is performed by IC 5, a further 74LS244. The data flow is of course in one direction only, out of the emulator and into the target system. When SWITCH is HIGH and the computer is in charge, part of IC 3 gates off the Enable to IC 5 completely.

Since the standard 2716 EPROM hat two select pins (chip Select and Power Down) which both must be low to access the EPROM, the remaining two NOR gates of IC 6 are included to simulate this function. Finally, capacitors Cl, C2 and C3 are included for supply rail decoupling. One buffer in IC 3 and two NAND gates in IC 7 remain spare. I thought of using one of the gates to drive an LED to show the state of the SWITCH control line.

Constructing.

Virtually any of the usual means of building circuits may be used, the original was built on a piece of plain matrix board about 5 inches square using hand wire-wrapping. It does not take too long this way but care is obviously needed to avaoid wiring errors as they are difficult to find later on. Before commencing wiring, the two disc ceramic decoupling capacitors C2 and C3 should be fitted and connected directly to the supply pins of ICs 10 and 11. The supply and ground connections are then dealt with followed by the address, data and control connections.

Connection from the Nascom to the emulator is via a 16 pin DIL socket, SKT 1 (shown by triangular connections on the diagram). Connections to the target hardware are via a 24 pin DIL socket, SKT 2, which is wired to correspond to the pinouts of a standard 2716 EPROM. The prototype uses a link between the 5 volt supply and pin 24 of SKT 2. This allows the target hardware to draw power from the emulator. Clearly, it has to be disconnected if the target system and the emulator have their own supplies!

The emulator is connected to the target system using a 12 inch length of ribbon cable fitted with a 24 way pin header at each end, wired pin to pin.