From: olson@anchor.esd.sgi.com (Dave Olson)
Newsgroups: comp.sys.sgi
Subject: Re: /dev/plp on PIs
Date: 7 Oct 92 04:43:15 GMT
Organization: Silicon Graphics, Inc.  Mountain View, CA

In <Bvpqzq.BFo@helios.physics.utoronto.ca> sysmark@helios.physics.utoronto.ca (Mark Bartelt) writes:
| the Personal IRIS Owner's Guide, page C7:
| 
|   Parallel Printer    Construction:     Centronics 25-line interface with
|                                         full shield attached 360 to solid
|                                         metal D shell; IBM PC compatible
|                       [ etc. ]

Hmm.  this wasn't caught in reviews; it definitely shouldn't have been
claimed.

| Well, at this point I was puzzled.  Then it occurred to me that, besides our
| eight PIs, we also have an Indigo, so perhaps I could try the printers there
| to see whether the Indigo exhibited the same behaviour.
| 
| At first, I was worried, because there is no reference to the parallel port,
| anywhere in the IRIS Indigo Owner's Guide, and I feared that perhaps Indigos
| came without a parallel port.  But, glancing at the back, I spied a connector
| (25-pin D connector), labeled "//", and deciding that this was intended to
| mean "parallel" rather than "C++ comment", I gave it a try.
| 
| Bingo!  The same printers which work fine on the PC clone and don't work on
| the PIs work just fine on the Indigo.

Did you perhaps connect the printer after the system was up and running?
We found a problem in the way some printers worked on the PI, which caused
us to add code to reset the parallel port interface on that system at
boot.  That fixed a number of printers.  I don't remember having seen
this problem on Indigos (which have a different chip for the parallel
port).  If you didn't reboot after connecting the printer on the PI,
you might want to try shutting down, connecting the printer, powering
it on, and rebooting.  You could also write a program that does the
PLPIOCRESET ioctl on the parallel port on the PI's to see if that helps
(without rebooting).

| So now, the question is why?  I suspect that there is some signal which the
| PI's parallel port hardware expects to see asserted, which the PCs and the
| Indigos don't care about.  Anyone care to hazard a guess as to what that may
| be?
| 
| The documentation isn't a lot of help.  The PI Owner's Guide doesn't give the
| pinout for the parallel port, as it does for the serial ports.  And, as I've
| already noted, the Indigo Owner's Guide doesn't seem to even acknowledge that
| the parallel port exists.  (Why?)

Apparently, an oversight.  The author is out sick, so I can't ask.

| The plp(7) man page gives a superficial overview of the electrical interface,
| giving mapping of signal names onto pins, but no description of just what the
| signals mean.  If I compare these with the corresponding documentation for an
| IBM (a real IBM) parallel interface, most of the signals correspond, but there
| are a couple minor differences.

I think I mentioned that fact in another posting.

| Most curious is the fact that the plp(7) man page says there's a signal called
| "RESET" on pin 31.  However, given that the parallel interface on both the PI
| and the Indigo is a 25-pin connector, I'd be interested in finding out which
| pin that signal is actually mapped to.

Pin 31 on the 37 pin end of the cable.  It is pin 16 on the 25 pin
connector.

| Any useful hints, from either SGIers or non-SGIers?  Although there's no great
| time pressure involved here, we will likely be getting a Phaser IIsd at some
| point, and it's quite possible that we'll want to hook it up to one of our PIs
| rather than to our Indigo.  TIA.

For what it is worth, the Indigo parallel port runs faster...
--
Let no one tell me that silence gives consent,  |   Dave Olson
because whoever is silent dissents.             |   Silicon Graphics, Inc.
    Maria Isabel Barreno                        |   olson@sgi.com