Re: 2764 -> 2364/68764 adapter

From: Gerrit Heitsch <>
Date: Wed, 13 Jul 2016 19:32:17 +0200
Message-ID: <>
On 07/13/2016 07:12 PM, Jim Brain wrote:
> On 7/13/2016 11:30 AM, smf wrote:
>> On 13/07/2016 13:48, Gerrit Heitsch wrote:
>>> In man circuits they control the EPROM through _OE and tie _CE to GND
>>> since that allows you to get away with slower EPROMs. This way the
>>> EPROM is always aktive and decoding addresses, just the output
>>> buffers are disabled. So you can hide quite a bit of the access time
>>> inside the delay for the decoder that selects the EPROM. Downside:
>>> higher power consumption with the old NMOS types. Doesn't really
>>> matter with CMOS.
>> Exactly, which is why I suggested hooking up to _CE and using a
>> buffer. I am assuming that anything with a 2364 pinout is going to be
>> the same speed or slower than something with a 2764 pinout.
> I think you have it backwards.  the OP wants to put a 2364 into a 2764
> socket, so the 2364 could be much slower than the original socket expects.
>> If the 2364 you are using is actually powered up all the time decoding
>> addresses and the _CE only turns on the output buffers
> _CE does not turn on output buffers.  _CE is chip select.  _OE turns on
> output buffers

Yes, but somewhere in a Commodore datasheet I read that that the 23xx 
ROMs didn't really have a power down, so they really might have had the 
ROM running all the time and using _CE to only control the output 
buffer. The datasheet mentioned their 24xxx ROMs being able to power down.

It's here, search for '24128'

>> then you could get away with just checking for (_CE | _OR ) == 0.
> You can do so, but that costs an OR gate, and most times, 2764 sockets
> just tied CE to ground all the time, as the power savings with CE =1
> would not matter.

With NMOS-ROMs it makes quite a bit of a difference. Replacing the 16KB 
NMOS ROMs in a Commodore C16 or +4 with a 27C128 each results in power 
savings of 50mA per Chip. 2 ROMs = 500mW power savings.

> If you
> deadbug the IC, you can stash it in the interior of the 24 pin socket,
> as you only need 5 connections (A,B,Y,PWR,GND)

Well, you should not leave unused inputs open. Some logic families don't 
like that.


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Received on 2016-07-13 18:00:33

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