80-Bus News


July-October 1982, Volume 1, Issue 3

Page 38 of 51


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 O 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:

1 DATA STROBE 19 Twisted pair ground for pin 1 2 DATA 1 20 Twisted pair ground for pin 2 3 DATA 2 21 Twisted pair ground for pin 3 4 DATA 3 22 Twisted pair ground for pin 4 5 DATA 4 23 Twisted pair ground for pin 5 6 DATA 5 24 Twisted pair ground for pin 6 7 DATA 6 25 Twisted pair ground for pin 7 8 DATA 7 26 Twisted pair ground for pin 8 9 DATA 8 27 Twisted pair ground for pin 9 10 ACK 28 Twisted pair ground for pin 10 11 BUSY 29 Twisted pair ground for pin 11 12 30 Twisted pair ground for pin 31 13 34 INPUT PRIME

14 System ground 32

15 33

16 System ground 34

17 Protective ground 35

18 36

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 O4H whilst its control port is port O6H, likewise, the B half input/output is port O5H and its control is port O7H. On the Gemini Multiboard machines port A I/O is port OB4H and its control is OB6H, port B I/O is port OB5H and its control is OB7H. 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.


Base address, O4H for Nascom, OB4H for Gemini Port A data 1/0 Port B data 1/0 Port A control Port B control

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3; Initialize the ports, port B to output, port a to bidirectional LD A, OFFH ; Force all outputs high, default condition is output. OUT (CENPAD),A OUT (CENPBD),A OUT (CENPAC),A ; Tell port A it will use the bidirectional/control mode LD A,OFDH ; Send the mask to say which bits are in and which are out

This is an OCR’d version of the scanned page and likely contains recognition errors.

Page 38 of 51