80-Bus News


September–October 1984, Volume 3, Issue 5

Page 10 of 47

The ports are used in a way compatible with the Gemini implementation of the Centronics interface, as follows:–

Port A is used in control mode.

Bit 0 is an output signal from the GIPB and is high when Busy and low when able to accept data.

Bit 1 is an input to the GIPB and is a strobe which goes low for a short time when data has been sent to port B.

Port B is used in control mode, as the GIPB input port. Bit 7 of the input data is ignored, and the output to the printer has even parity added to follow the normal standards.

Operation of the system is completely automatic, and all data received is printed as soon as possible. The program uses a circular buffer and compresses consecutive spaces to save memory. Up to 128 consecutive spaces can be held in one byte. Most listings contain many spaces, so the buffer will often be able to hold well over 100K in the 60K available. If the buffer becomes full the Busy line remains high so that no data can be lost.

If a keyboard is attached the following single character commands are available:–

SpaceHalt the printer, or if halted start printing again. This does not affect the input of data to the buffer.
DDelete the contents of the buffer and restart the program.
TDelete the contents of the buffer and restart the program with a minimal buffer of only two bytes.
CROutput CR/LF to the console device (normally the printer).
MOutput a status message to the console device. This shows the number of characters waiting to be printed, the number of bytes spare in the buffer, and whether or not the printer has been halted.
NEnd the program and pass control to RP/M. RP/M operates as normal and can boot a disk system but does not have any cassette handling routines. The command G F000 will execute the program from RP/M.
!Halt the processor.

Technical Notes

The GIPB operates on an interrupt driven basis, with an interrupt being generated when the input strobe goes high rather than low. It was necessary to do it this way because some host software does not initialise the ports correctly so that the first character is lost following a Reset. This method overcomes this problem and should not cause any problems. Some host software will send a null during initialisation. This is sent to the printer which is likely to ignore it.

The GIPB catches all characters transmitted by enabling interrupts and then setting the Busy line to 0. After about 8 instructions the Busy line is set back to 1. This should give the host machine plenty of time to notice that the line is not Busy, and decide to output the data. The GIPB waits for

Page 10 of 47