TTL computing

From: Richard Erlacher <>
Date: Fri Apr 12 10:54:45 2002

see below, plz.


----- Original Message -----
From: "Ben Franchuk" <>
To: <>
Sent: Friday, April 12, 2002 8:39 AM
Subject: Re: TTL computing

> Richard Erlacher wrote:
> > Using a PROM to implement a combinatorial path has always been the most
> > way to do it. I don't think it was as common as you suggest. Such an
> > application is likely to lead to very sparse ROM utilization. The ROM has
> > fixed Or and fixed AND array, hence has to have many more registers in it
> > a programmable logic device capable of the same logic, and, likewise, many
> > more than the equivalent logic implemented discretely in SSI/MSI logic.
> How ever the ROM does contain ALL the possible states compared to PAL's
> or discrete logic.
It doesn't help having all the unused states represented in your ROM.
Discrete implementations and programmable devices both cover all the REQUIRED
states, and having other states supported is certainly unnecessary, as well as
potentially hazardous.
> BTW B.G micro does sell 74LS170's, and AM2901's.

I've not needed a '170 in over two decades. I do have a supply of '670's,
which are the tristate version of the same device, and I just this week
finished an application using a couple of them in a display application.
They're pretty handy when you have no synchronization between the inputs and
outputs, and, as was the case in this 7-segment display device, when your
inputs are byte-wide, and outputs are nybble-wide.
> --
Received on Fri Apr 12 2002 - 10:54:45 BST

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