80-Bus News

Using this interrupt mode, which is in general known as “Vectored Interrupt’, we are more or less obliged to use the Z80 family of peripherals to allow us to avail of the vectoring. It may be possible to use other devices to the same effect, but this will depend on the device. Zilog make two families of peripheral devices. Designed as full members of the Z80 family are the following:

In addition to these devices, Zilog also offer another family, the 8500 peripheral device family. These devices are specialised microcomputers, which are set up to be input/​output devices of astonishing complexity! I propose to deal only with the PIO and CTC at present, as these are the only two devices on which I can claim any experience. The SIO and DART I may be able to deal with in the future as Gemini have an SIO board in design. [Ed. – using SCCs from the 8500 family, I believe, not SIOs.] The DMA chip presents problems when using an N1 with Nascom bufferboard, as it becomes necessary to modify the buffer board to allow the DMA to reach into the 4k block of the standard Nascom.

Let us now park the CPU interrupt handling to one side, and consider first the PIO. This device offers a number of operating modes. It comprises two 8 bit parallel ports, each with two handshaking or control lines. As normally used, it appears as a block of four ports, with the data ports A & B, and then the corresponding control ports CTRLA & CTRLB. On the standard Nascom, this block occupies Ports 4,5,6,7. Port A is number 4 with its control port CTRLA number 6. The two data ports are Read/​Write, the two control ports are Write only. We can set this device up in a number of operating modes. First of all, we can pick Mode 0, which configures the port as Output, say driving a parallel printer. The two handshake lines are used here, one from the PIO to indicate that the data is ready on its output pins, and the other from the peripheral device to say ‘Thank you, I’ve got that’. Mode 1 sets the port up as Input, with the handshake lines working in a similar way. These two modes can be set up on ports A or B, but mode 2 differs. Mode 2 sets up port A to handle bidirectional data. As it now needs two hand shakes, one for in data and one for out data, we steal the handshake lines from Port B. So Port B can only be set to Mode 3 if port A is in Mode 2. Either or both ports can be set to mode 3 at any time – it is not necessary to have a port in Mode 2. Mode three is the Bit Mode. It allows the eight lines from the Port to be configured as any combination of Input and Output lines you require. As well, we may instruct the port to interrupt on simple logical combination (AND or OR) of some of these lines. We don’t have to use them all for the interrupt, and can also define whether they should cause an interrupt when high or when low. There is one restriction – all interrupting lines must be at the same level.

On first introduction setting the PIO up is so complex, that an example is perhaps the best way to demonstrate. We will set the PIO up to its four modes by way of example.

On power up, the PIO enters a reset state. This means that the internal structures of the PIO are all set to zero, or neutral conditions, as appropriate. We unfortunately cannot get this effect by hitting System Reset on N1s, as the PIO hadn’t enough pins to allow for this. Instead, the PIO designers provided that a reset would be signalled if the PIO received an /M1 signal without /RD or /IORQ. To obtain this signal, it is necessary for N1 owners to make a small