Re: IEEE-488 cart (was Re: D9090 (was Re: VIC-1001 ROM sighting))

From: Ruud Baltissen (
Date: 2001-09-20 08:14:54

Hallo Ethan,

> But how are the specs?  It's an easy-to-get chip, but is it _really_
> a replacement, or is it just close?

The MC3446 is nothing more then a Open-Collector buffer going out and an

           | \
B 0--------   -----+
           | /     |
           |/      |
                   +---------0 X
             /|    |
            / |    |
A 0---------  -----+
            \ |

The main difference of these buffers and normal TTL-buffers is their
behavior when powered off. With TTL-buffers you run the risk that they tend
to pull the lines low when powered off, the IEEE-buffers won't. Not that
much but it is measurable. Connect several 1541s to a C64 but leave one
unpowered. You most probably won't notice it. Power off more and there will
be a point that the chain won't work anymore.

For techies amongst us: the pull-up resistors have their share in this
behavior as well. And talking about pull-up resistors: the 3446 also
contains such a resistor PLUS a diode. This diode makes sure the resistor
start pulling the line (L) when no power is applied.

Take as
example. UB16, a 6520, takes care of the databus of the IEEE-bus: Port A
reads the data, port B outputs it.

I cannot do without the OC-buffer. That's why I need the 7407. This is a
very common TTL-IC and will cost about 50 dollarcents ??? At least we're not
talking about dollars. As Port A already serves as an input, we don't need
one. If you want a kind of protection for your 6520, you could place a
74LS541, an 8-bit buffer, between the bus and the 6520.

FYI: I connected a bi-directional PC LPT-port directly to a 8032 and 4040
and used the PC as a second drive without any problem.

> If so, then you are probably limited to one or two peripherals.

What makes you think so? The IEEE-bus has much more then a normal fan-out (=
output capacity). I don't know the fan-in (= input load) of the 6522 right
know but it is peanuts for the bus.

  / __|__
 / /  |_/     Groetjes, Ruud
 \ \__|_\

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