Re: HPIB bus addressing (was "bus commands")

From: Larry Mollica (
Date: 2004-04-19 19:42:11

Quoting Marko Mäkelä <>:

> On Mon, Apr 19, 2004 at 12:45:47AM -0700, Larry Mollica wrote:
> > One interesting tidbit, the OPEN statement allows 4-31 as IEEE488
> > addresses, with 0-3 used for PET internal devices. But we found (ok,
> > by accident), that you can address IEEE488 addresses below 4 by using
> > values over 31, like
> > OPEN 1,34
> > to talk to IEEE488 bus address 3.
> Wouldn't that access device 2 (32+2=34)?

Yes, my mistake. I was up way too late last night.
I recall seeing the HP address dip switch set on 2.

The long version:
My buddy is a retired engineer and a bit of a computerphobe. He is just now
learning BASIC on the PET. He asked me to look over a program he'd done to
control an HP switch box. When I saw

I said; hey, that can't work! But it was working and talking to the HP box. I
asked him how the hell he got to 34. Well, the HP manual had a table that
looked something like:

  Switch    ASCII
  00000     [space]
  00001       !
  00010       "

For some reason this particular HP manual represented the bus addresses as
printable ascii, no mention of the 6th bit! So he looked up the ascii value for
quote, and used that value. He had no idea it wasn't supposed to work,
especially since it did work.

I have no idea why HP did that. Like I say, their manuals are all over the place
when it comes to the subject of remote control. I've looked at a few and this is
the first one I recall seeing that has a table like that.

OT; some of the earlier HP gear manuals have examples of sending to the
instrument via IEEE488 from a punch card reader. The card has holes for 7 data
bits, the 8th bit is labeled ATN.

Anyway, I'm not advocating using the PET trick to get to bus addresses below 4,
just making the observation that it seems to work, at least on PET ROM 4. We
are going to stick to addresses 4-31, unless the need arises to do otherwise.
Which seems unlikely.


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