E&MM's music control microperipheral is already compatible with the Acorn Atom and Spectrum micros, and this month we present two alternative methods of modifying the original design for use with the BBC Model B. Jim Grant and David Burden

Next month's E&MM will feature an extensive drum sequencing program that makes use of our OMDAC project, analogue electronic percussion modules (see elsewhere this issue for details of how to turn these into a complete electronic drum kit), and the BBC Model B home computer. However, since the OMDAC was originally designed for use with the Acorn Atom and Sinclair Spectrum micros, some hardware modifications must be carried out before it can be controlled by the Beeb.

Both approaches make use of the BBC's 1MHz bus and the memory page known as 'FRED'. In the first method address line A7 is used to select between the ADC and the 8255 PIA. Of course, this is a terrible waste of I/O space, but that doesn't matter too much if nothing else is mapped into page FRED. Due to the peculiarities of a 2MHz processor controlling the 1MHz I/O bus, Acorn recommend that the circuit shown in Figure 1 be used to 'clean up' the FRED page select. The new addresses are shown in Table 1.

Figure 2, meanwhile, shows a simple circuit for generating separate R and W signals for the OMDAC. This is necessary because the 6502 uses a single R/W line while the 8255 PIA was intended for the 8080/Z80 microprocessor family. The best way to incorporate the BBC decoding circuits is to build everything on a piece of stripboard and mount it on pillars inside the case. Remove ICs 16 and 17 and solder flying leads from the strip-board to the OMDAC PCB.

The alternative method involves only a simple rewiring of the logic gates so that they respond to the new addresses. The modifications required are shown in Figure 3. These put the DAC/VIA side of OMDAC at &FC00 - the bottom of FRED - and the ADC at &FC80.

However, it should be noted that the decoding is not complete, and that further work would be required if any other device were to be used on the 1MHz bus at the same time. To communicate with OMDAC, an OSBYTE call (with A% = 146 to read or A% = 147 to write) is used. X% holds the offset within the page (0-3 for the VIA registers, 128 for the ADC) and Y% holds the data.