8 Jan
2001
8 Jan
'01
5:23 p.m.
Some computers generate stb/ using a monostable
that's triggered from the
clock signal to the output data latch in the computer. This means that
one edge of the strobe pulse is effectively coincident with the change of
the data lines to the printer. And I've seen _both_ polarities of pulse
used on so-called 'centronics' ports -- it appears that the printers were
all edge-triggered (at least modern printers are), but which edge is the
important one depends on the whim of the designer.
Yeah, well, ACK and BUSY are pretty much at the whim
of the designer, as well. I really don't remember all
the fun we had trying to come up with a state machine
to give us an interrupt when the printer had accepted
the last byte we gave it and was ready to accept
another.
I seem to recall things like some printers hold ACK
normally deasserted then pulse it asserted to signal
they've accepted the byte while others do it the
other way around. And you can't count on the
relationship between BUSY and ACK pulses, either,
although IIRC PCs tend to ignore one of them (I
don't recall which one).
For such a simple interface, Centronics sure is
a mess...
--
Roger Ivie
rivie(a)teraglobal.com
Not speaking for TeraGlobal Communications Corporation