This month we present a complete general-purpose Z80 microprocessor board, called the MPC (Memory-Processor-Clock). The purpose of the MPC is to take the hard work out of building microprocessor-based projects. The board contains a Z80 cpu, clock, memory and I/O decoding logic, up to 4K of static RAM and 8K of EPROM. This makes it suitable for most small-to-medium sized applications.

Whereas most processor boards are intended to be used as part of a system in a card rack or with cards plugged into them, the MPC is designed to function as a 'programmable component'. Its first application is the 'Electric Drummer' project following this article.

Circuit

The circuit diagram of the MPC board is shown in Figure 1. Memory address decoding is provided for the following ranges:

MEM0 - 0000 - 0FFF
MEM1 - 1000 - 1FFF
MEM2 - 2000 - 2FFF
MEM3 - 3000 - 3FFF
MEM4 - 4000 - 4FFF
MEM5 - 5000 - 5FFF
MEM6 - 6000 - 6FFF
MEM7 - 7000 - 7FFF

MEM0 addresses ROM1 (IC 12), MEM1 addresses ROM2 (IC 13) and MEM2 is decoded to address the four pairs of 2114 RAM's as follows:

IC4-5 : 2000 - 23FF
IC6-7 : 2400 - 27FF
IC8-9 : 2800 - 2BFF
IC10-11 : 2C00 - 2FFF

MEM3, MEM4 and MEM5 are taken to edge connector P2 for external memory expansion if required. I/O decoding is also done on the board and all the decoded select lines (I/O 0 to I/O 3) are taken to the edge connectors.

The on-board clock, (IC17) is configured so that any combination of crystal and capacitors may be used. Also, potentiometers may be inserted for adjustment of frequency and range. These should be linked to ground if a crystal is used. Manufacturers of the 74LS124 recommend that V_cc should be less than 5V and provision is made on the board for the insertion of one or two diodes to give the required voltage drop.

Links are provided on the board for the selection of:
(1) Main or auxiliary +5V supplies for the RAMs: the auxiliary supply may be used to drive low-power CMOS RAMs.

(2) 2716/2732: each ROM socket may be individually selected for a 2K or 4K EPROM.

The edge connector details are shown in Table 1. P1 supplies all the signals required to drive up to four Z80 I/O devices (including interrupt vectoring) or other I/O. All other signals are taken to connector P2. Thus, for most small applications only connector P1 is needed. In most cases, if external devices are kept close to the connector the busses do not need to be buffered. Thus the cost of adding 'intelligence' to a system is kept to a minimum.

Construction

The PCB is double-sided and its size has been kept to 3¾" x 7½". This means that if the board is plugged into another PCB the height of the box required is still reasonable. Initial versions of the board are not through-plated and some 200 through pins must be inserted in the board and soldered both sides. However, if time is available this should present no problem in construction.

Maplin's part number for the PCB is GA56L. No specific construction details are given, since the circuit configuration may be changed to suit the application. Part 2 of the 'Electric Drummer' project will contain details for using the MPC board with this unit.