80-Bus News

The Centronics connector is different to the D25 connector. It is a 36 way connector arranged so that all signal leads are on the top row, whilst the opposite pins on the bottom row are all signal grounds. This allows for the use of twisted pair cables to improve the noise immunity. To confuse the issue, Centronics refer to the eight data lines as DATA 1 thro’ DATA 8, whilst computer people think of these as Bits 0 thro’ 7. DATA 1 is connected to BIT 0, DATA 2 to BIT 1, etc. Beware, as has already been noted, there are some variations on the theme, Epson for instance use one of the lower pins as a device select pin on some models, so it pays to read the manual referring to an individual printer before connecting up. A typical Centronics configuration is as follows:

The DATA lines are connected to the PIO port B, bits 0 to 7, the STROBE is connected to PIO port A, bit 1, whilst the BUSY line is connected to PIO port A bit 0. The warnings given earlier about system and protective grounds apply equally in the Centonics interface, so care is advised.

Again, as with the serial output, the scheme is simple and follows similar lines. However, the PIO device must first be primed. The PIO has several modes of operation, input, output, bit mode and bidirectional/​control mode, and uses a control port for each half of the PIO device to program it’s respective mode. The control port for the PIO is addressed at the port address + 2. On a Nascom the actual input/​output port for the A half of the PIO is port 04H whilst its control port is port 06H, likewise, the B half input/​output is port 05H and its control is port 07H. On the Gemini Multiboard machines port A I/O is port 0B4H and its control is 0B6H, port B I/O is port 0B5H and its control is 0B7H. If we use labels for these addresses, the following listing will make sense regardless of the actual port addresses. The initialization routine need only be called once prior to use, but note that RESETting the computer will return the PIO device to it’s default mode, and the routine will need to be called again. These routine are due to Richard Beal (again) as they are more elegant than my original version, although somewhat more complex.