That should explain itself, there is nothing complicated about it, it uses the Nascom
output routines to do the hard work. Note that SRLX is called from within the
handshake program, this means that it is not necessary to set the X command on,
although the last parameter given to the X command will be the default protocol for
the output. If the X command is to be used, simply omit the RCAL SRLX, and the routine
will work as a handshake routine only.
Driving a printer fitted with a Centronics input is a little more complicated.
As has already been mentioned, the Centronics output is parallel so use is made of the
Nascom or Gemini onboard PIO
device. The PIO device ought to be the subject of an
article in it’s own right. Howard Birkett covered it way way back in
INMC 1 or 2,
and I had a go at describing it in
Although these magazines are long since out of
print, Howard’s article is reprinted in the ‘Best of INMC’ which is still available. I
will not dwell on the PIO device itself, simply list the actions needed to prime the
device to make it work as Centonics type interface.
Before starting to write an interface program it is necessary to understand
what it is it is intended to drive. The Centronics interface works like this.
|1||seven bits of data are placed on the output|
|2||A slight delay to allow the data to settle|
|3||A strobe pulse is generated to cause the printer to latch in the data|
|4||A slight delay to allow the data to be latched|
|5||The strobe pulse is removed|
|6||The printer generates an ACKnowledge pulse|
|7||The printer also generates a BUSY signal which remains active until any print
action is complete.|
1. When data are accepted:
|T1 ~ T3:||1µ sec min.|
|T4:||100 nsec max.|
|T5:||0·1 ~ 0·5 msec|
|T6:||6 ~ 8 µsec|