Re: 6569 luminances

From: Gerrit Heitsch <>
Date: Mon, 14 May 2012 18:22:00 +0200
Message-ID: <>
On 05/14/2012 06:02 PM, MikeS wrote:
> ----- Original Message ----- From: "Gerrit Heitsch"
> <>
> Sent: Monday, May 14, 2012 6:56 AM
>> As for the ROMs... you could desolder all of them, and add a little
>> adapter with a 74LS11 and a properly programmed 27C256 on it.
> -------
> One of Jim Brain's 2364 adapter boards ($1.50, $5.00 assembled) is ideal
> for
> this; just add some diodes and a pull-up resistor to wire-AND the /CEs.

I don't like the wired AND and prefer to use a logic chip since that 
will make sure the _CS-line for the ROM goes inactive as fast as it goes 
active (LOW). Watch what a pullup does to a signal on a scope and you'll 
see what I mean. A good example being the R/_W line on the C64 board the 
moment VIC uses AEC to remove the CPU from the bus. And that one has a 
1.5KOhm pullup!

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 when connecting 
them to the control signal from the board.


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Received on 2012-05-14 17:00:35

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