Scor­pio News

advanced. The Company that I work for (AIRO) uses a small number of 80-BUS products as controllers for some rather specialized audio systems that we design and build for Concert Halls and similar buildings in this Country and overseas. We also use Gemini machines in a network for our in-house administrative functions, word processing etc.

For a current project for an overseas client we needed to incorporate an audio frequency oscillator with programmable frequency control to be used for installation and subsequent calibration of the system. We decided that this could best be achieved by building the oscillator into the control computer itself as this would provide us with the flexibility to use the host controller for a number of ancillary tasks and to reduce the amount of equipment that we will need to take on site with us for the installation work. We therefore commissioned the manufacture of an A/D and D/A board that would give us the required level of control in both the time and frequency domain.

The digital design and production of the board were entrusted to CAT Systems of St Albans who have considerable experience with DSP equipment while AIRO was responsible for the audio circuitry. The resulting “board” needed to operate quite quickly and to have some intelligence of its own and the designers chose to use the Texas Instruments TMS 320C25 digital signal processor chip running at 25 MHz and driving a 14 bit D/A convertor (DAC 1200KP- V) and complementary A/D device (TDA 1534). The design effectively provides a DSP co-processor board with a Z80 control interface. It is equipped with its own 64 k of 16 bit static RAM and EPROMS to contain software that can be transferred to RAM if required.

The Z80 control interface uses a PIO to drive essential control lines and a DMA to transfer commands, data and programs into the TMS memory space or to read information back. Although the DMA is a horrible device to program this approach does allow the Z80 and TMS processors to operate asynchronously.

The Z80 DMA is only a 16 bit device and to enable transfers to be made to and from the full 64 k x 16 bit TMS memory space and 512 k x 8 bit area allowed for in the 80-BUS specification, 4 bits of the PIO are used to latch the high order address lines when the DMA is bus master. This means that the full capabilities of the 80-BUS range of products are unaffected by this design. However, you do have to know what you are doing. We discovered the hard way that using a GM813 or GM811 combined with a 64 k dynamic RAM board works very nicely with the memory at Page 0, but if the GM811 is teamed with a static RAM card (GM 863) you may think you are using the default Page 0 whereas in fact you are using Page 7. This did take quite a time to sort out out as unfortunately at the time we did not have the circuit for the RAM card!