Apple II Computer Info
printer manual implies that pin 1 is on the right when looking into the
printer connector, thus on the left when looking at the plug on the
cable. This would match a DB-25 (male on the plug, female on the
printer).
To be on the safe side, plug the cable into the computer but not the
other device, make sure the computer is switched off, and use a
continuity tester to locate the block of ground lines at one end of the
bottom row of pins.
When using a parallel cable with a PC, you have access to all the
printer control and status signals (STROBE, ACK, BUSY, PAPER EMPTY,
SELECT, AUTO FEED XT, PRIME, ERROR and SELECT). With an Apple II, a
fair number of these signals may not be available, depending on the
printer card.
For example, Apple's parallel interface card has eight outputs (plus
strobe, which is generated automatically) and eight inputs. I don't
know offhand where the inputs are supposed to be mapped, and can't
locate the manual for the card just now, but the lack of additional
outputs prevents control of AUTO FEED XT, PRIME and SELECT ENABLE.
The standard handshake logic for the parallel port is as follows:
1. The computer outputs 8 bits of data.
2. There is a minimum 0.5 microsecond delay before activating STROBE.
3. STROBE is pulsed low for at least 0.5 microseconds.
4. The printer BUSY signal goes high in response to strobe, within a
maximum of 0.5 microseconds.
5. There is a minimum 0.5 microsecond delay before the data can be
removed.
6. Once the printer has finished processing the data byte, it lowers
BUSY again, then generates a low-going pulse on the ACK line. This is
delayed by 4 microseconds after BUSY went high, and is approximately 3
microseconds wide.
The printer drives BUSY in response to STROBE, but I'm pretty sure the
computer doesn't wait for BUSY before it sets STROBE high again.
The computer usually watches BUSY to identify when it is able to send
the next byte. It may ignore the ACK signal completely. Some computers
rely on the ACK signal, using a hardware latch to ensure it is not
missed, possibly in conjunction with an interrupt trigger. If the
computer's parallel port has an output buffer, the ACK signal would
initiate transfer of the next byte.
The PAPER EMPTY signal goes active (high) as an advance warning of paper
empty. If printing continues, the ERROR signal goes active (low),
followed by the SELECT signal going inactive (low).
The SELECT signal can also go false if the printer is deselected.
The PRIME signal is pulsed low for a minimum of 1.0 microseconds to
reset the printer.
--
David Empson
Snail mail: P O Box 27-103, Wellington, New Zealand
Apple II Computer Technical Information : Apple II Family Hardware Info
ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 332 of 572
(22 pages)
(16 pages)
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