----- Original Message ----- From: "Gerrit Heitsch" <gerrit@laosinh.s.bawue.de> Sent: Monday, May 14, 2012 12:22 PM Theory vs. practice... > 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). Granted, in theory it's better to have an active pullup to drain the input capacitance than a resistor, but in practice I've used diodes (with 10K pull-ups) with *CMOS* EPROMs at 1 to 4 MHz speeds in dozens of applications and never had a problem; it's much simpler, and it was after all the way it was done in the days before TTL... > Also, almost all 2364 to 27xxx adapters have the same design flaw, they > tie _CS for the EPROM to GND, ...eating power... Well, again in practice, considering that just *one* of the 2332/2364s you're replacing draws ~100ma (and doesn't have a low power standby mode AFAIK) and the 27Cxxx only draws around 25ma, is this really an issue? If you're really anal about it you could of course just cut & jumper /CE & /OE. m > On 05/14/2012 06:02 PM, MikeS wrote: >> ----- Original Message ----- From: "Gerrit Heitsch" >> <gerrit@laosinh.s.bawue.de> >> 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. >> http://store.go4retro.com/2364-adapter/ > > 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. > > Gerrit > > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2012-05-14 18:00:15
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