Re: ROMs replacement

From: Gerrit Heitsch <>
Date: Sun, 16 Sep 2012 12:57:23 +0200
Message-ID: <>
On 09/15/2012 11:12 PM, wrote:
> On 2012-09-15, at 18:12, Gerrit Heitsch wrote:
>> A second charset means a [...] more complicated circuit
> Something like this?

Doesn't look quite right to me...

Lets recap what you plan on doing:

If _CharROM is HIGH, A12 must be passed to the EPROM unchanged, 
otherwise BASIC and KERNAL run into problems.

if _CharROM is LOW, you want to be able to control its level by a jumper 
or switch to allow 2 charsets.

The simplest way to achieve that is a 74LS257. Tie _OE to GND, connect 
_SELA to _CharROM. A12 goes into I0b, your switch goes into I0a and A12 
of the ROM is connected to output Za. Tie all unused inputs to GND. 
Don't forget the pullup on I0a. This also makes sure that the one pullup 
you need is again on a static signal.

The LS257 is a bit overkill, but it does the job with a single IC and 
one pullup and you can be sure that all logic levels are correct.

A 74HCT257 might also work.


       Message was sent through the cbm-hackers mailing list
Received on 2012-09-16 11:00:39

Archive generated by hypermail 2.2.0.