Re: IEEE488 electrical specs

From: Martijn van Buul (
Date: 2001-04-18 14:00:09

Ruud Baltissen wrote:
> Hallo Martijn,
> > Does anyone have the *electrical* specifications of the IEEE488 bus?
> I have to dig up the exact info at home. But AFAIK you should recon with
> about 3 Kohm for every line. 
.                        ^^^^^
Shouldn't that read 'device' ?

> This means 15 mA is sufficient enough to drive 8 peripherals.

Okay, that's enough for the time being. It's not completely according to
the official standard, but I can live with that :)

> I assume you intend to use every portpin in bidirectional mode.

Yes. Saves a *lot* of I/O pins. It would be Real Nice (TM) I could squeeze
it into just one 8751 and nothing else save some glue.

> I thought I may have some 8751's laying around, but I'm not able to program
> them.

Programming a 8751 isn't really difficult. I have the specifications floating
somewhere on my desk at Philips. Interested in a copy? 

> Could these be of some help for you?

Well, I use a 89c51RD+ for my 8051 development. That thing has 64K flash, 
which can be programmed over an RS232 connection. Rather convenient. It's
a shame that this cute chip isn't available at electronic part resellers.
(Mail-order companies like Conrad have it, though).

> Questions:
> - Is it going to be bi-directional? (My Tripod lacks this feature :( )

Well, it's a bit too early to make any sensible remark about this :) It
would be nice, though. I haven't completely figured out how I want things
to work exactly; I don't want to make it a 'transparant proxy', for that
would slow things down tremendously. Suppose you have a PET, a diskdrive
(4040, 8050, whatever), this converter and a 1541. It would be a shame if
the connection between the PET and the 4040 would be slowed down because
of the 1541..

> - As you only need about 20 lines, 4 are left over. What a an extra RS232
>   port?

Hmm. I don't really follow you here. A 8751 has 4 I/O ports (thus 32 IO
lines). However, I think that the end result will probably consist of
a 8031 (the ROMless version of a 8751) with an external EPROM. In that
case, you loose 2 ports because of the address/databus which sits at
ports 0 and 2. (It may be possible to reclaim a few IO lines though, 
I don't think that you'll need the full address range of 64K)

Regarding an extra RS232 port: The 8751 has a built-in RS232 port; if
you want to add a *second* one, you'll end up bit-banging such a thing.
I'm not quite sure if that's possible, since the IEC-bus has to be
bit-banged as well. (hmm. Maybe not; maybe I can abuse the UART for that..)
Anyway; thanks for the info :)

    Martijn van Buul - -
	 Geek code: G--  - Visit OuterSpace: 3333
   Kees J. Bot: The sum of CPU power and user brain power is a constant.
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