The circuit diagram of the control board is shown in Figure 2. P1 is the edge socket for the MPC board interface connector.

IC5 is connected to the processor's maskable-interrupt input (int) to control the timing of the system. A 10-turn potentiometer on the front panel gives fine tempo control.

Input/Output is controlled by IC1, 2 and 3. ICs 1 and 2 give 24 lines each of general-purpose I/O lines; 16 lines are used for driving the instrument triggers, 16 drive the front panel lamps, two are used for cassette I/O, one for downbeat indication and 12 lines for scanning the control keys (see E&MM September 1981, p42). IC3 is a multiplexing driver for the Electric Drummer's four-digit LED display.

The I/O addresses are as follows:

IC 1:	Port A	40H
	Port B	41H
	Port C	42H
	Control	43H
IC2:	Port A	80H
	Port B	81H
	Port C	82H
	Control	83H
IC3:	00H	05H

The trigger outputs are standard +15V pulses. The cassette interface is implemented without the use of a USART; all timing is carried out by the processor, and to ensure correct operation, a software error checking system is used.

As no PCB will be produced for this project, it is suggested that it is constructed on DIP board using Verowire or wire-wrap. It is, of course, possible to put the processor and memory on the same board, but the use of the MPC board makes the task far easier. Remember to include several 10nF decoupling capacitors.

Two options are available for connecting the board to the front panel and trigger output sockets. The cheapest method is to use Veropins on the board and wire each to the appropriate lamp/switch. As there are so many connections, however, you may prefer to use edge connectors and ribbon cable for this purpose.

Power Supplies

Figure 1 shows the suggested power supply. All voltage regulators should be mounted on heatsinks or, if a metal cabinet is used, they could be mounted on the back panel (using mica washers for insulation, of course). The 0.47uF and 0.1uF capacitors should be connected as close to the pins of the regulators as possible. The -15V supply is required for the sound generator board, and may therefore be omitted if the Electric Drummer is used only to drive other equipment.

In the next article we will look at the optional battery backup circuit, and at a suitable sound generator board to use with the Electric Drummer.