Re: 6569 luminances

From: Gerrit Heitsch <>
Date: Mon, 14 May 2012 19:34:31 +0200
Message-ID: <>
On 05/14/2012 07:05 PM, Jim Brain wrote:
> On 5/14/2012 11:22 AM, Gerrit Heitsch wrote:
>> Also, almost all 2364 to 27xxx adapters have the same design flaw,
>> they tie _CS for the EPROM to GND and use _OE to control it. This
>> works obviously, but it keeps the EPROM active all the time, eating
>> power, since _OE only controls the output drivers. Unless you want to
>> use very slow EPROMs and need the extra few ns a grounded _CS will
>> give you, the proper way is to tie _CS and _OE of the EPROM together
> It might be proper, and I considered it on the 2364 and 23XX adapter,
> but I chose not to implement for a few reasons:
>   * I didn't think the extra power requirement would be an issue.

With a CMOS-EPROM it's not much, but there are still a few NMOS-EPROMs 
around that could get used and with them it makes a difference, at least 
in heat output.

>   * I was worried that the delay of enabling the ROM and it's output
>     drivers would cause timing issues. On a 250nS 27C64, it can take up
>     to 350nS from !CE to data valid. That's compared to 120nS for !OE to
>     data valid.

Where did you get the 350ns for a  250ns EPROM? I'm looking at the 
datasheet for a 27C64 from NS and that one states that for a 250ns type, 
  tCE (_CE to output delay) is 250ns and 350ns for a 27C64-350. In that 
datasheet the times are supplied in a table and can be misread. Since 
you mentioned 120ns (which is tOE for the 27C64-350, the 27C64-250 has a 
tOE of 70ns), you might want to take another look at that datasheet you 

> If folks are sure the two can be tied together, please let me know.

I have done it and didn't get any complaints from the circuits I used 
that in yet, namely 1541 and C64.


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Received on 2012-05-14 18:00:06

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