Re: Apple II Centronics

[dempson@actrix.gen.nz (David Empson)]

Jeff Blakeney <CUTblakeney@home.com> wrote:

> On Tue, 16 Nov 1999 00:55:52 +1300, dempson@actrix.gen.nz (David
> Empson) wrote:
> 
> >Nicolas Rey <nicorey@club-internet.fr> wrote:
> >> printer) with a custom BASIC program. So far, I've used on/off feature
> >> of the serial ports (through PR#1 and PR#2) to command a two relays. I'd
> >> like to have more controls and I'm considering switching to a IIe using
> >> the parallel port.
> >
> >Apple IIc?  There aint no parallel port, I'm afraid.
> 
> Tsk, tsk, David.  You should have read his message more closely.  :-)

Whoops!  I saw "IIc" and "parallel port", and the rest of the article
went out the window.  :-)


OK, lets backtrack and try again.

Nicolas Rey <nicorey@club-internet.fr> wrote:

> I'd like to have more controls and I'm considering switching to a IIe
> using the parallel port. I couldn't find the pin asignment of the
> Centronics plug in my documentation.  Any one still has that information
> or knows where it can be found on the web ?

Given an Apple IIe and a parallel card, I suppose you could set up an
external device with a female centronics connector, and plug the
parallel cable into it from the IIe.

Another option (possibly cheaper) would be to bypass the parallel cable
and connect directly to the DB-25 provided by the parallel card.  The
disadvantage with this method is that the pinout of Apple II parallel
cards are different for different brands.  The Grappler is similar to
the PC's parallel port, but most other cards are completely different.
The cable that is supplied with each parallel card will usually only
work with that card.

I'd suggest sticking to the Centronics connector for now, since it lets
you switch to another parallel card (or even another computer) without
having to modify your cable or circuitry.

The Centronics pinout is documented in most older parallel printer
manuals.  Off the top of my head, I can remember the following:

 1  STROBE (active low) from computer
 2  Data bit 0
 3  Data bit 1
 4  Data bit 2
 5  Data bit 3
 6  Data bit 4
 7  Data bit 5
 8  Data bit 6
 9  Data bit 7
10  ACK (active low) from printer
11  BUSY from printer

I've dug out a Panasonic printer manual, and it lists the rest of them
as follows:

12  PAPER EMPTY (output from printer)
13  SELECT (output from printer)
14  AUTO FEED XT (input to printer, active low)
15  (unused)
16  Signal Ground
17  Frame Ground
18  +5V (output from printer, reference voltage only)

Pins 19 through 29 are ground lines, paired with each of the STROBE
through BUSY lines (for use in a twisted pair or ribbon cable).  Pin 30
is the ground return line for pin 31 (PRIME).

31  PRIME (reset input to printer, active low)
32  ERROR (output from printer, active low)
33  Signal Ground
34  (unused)
35  (unused)
36  (unused)

I happen to remember that some printers use pin 36 as a SELECT ENABLE
input, but I don't recall the polarity.

All signals are TTL-compatible, with rise and fall times less than 0.2
microseconds.

The pins are numbered in the same manner as a DB-25 - along the top row
and then in the same direction along the bottom row.  What I'm not
certain about is which end has pins 1 and 19.  The diagram in this
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
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand