EPROM Substitution

[Tony Duell]

> 	The pinout between the two chips is slightly different, thanks to the 
> fact that the 2532 has only one enable line and the 2732 has two. I've 
> come up with an idea for conversion, but I'd really like to get a sanity 
> check on it, preferably from someone with an original TI data book that 
> covers the 2532. 
> 
> 	Here's what I've got so far, based on comparing the pinouts of the 
> two devices.
> 
> 	(1) Move the trace originally connected to pin 18 (A11) on the 2532 
> up to pin 21, which appears to be A11 on the 2732. 
> 
> 	(2) Tie pin 18 to pin 20 so that, on the replacement 2732, both /CE 
> and /OE are asserted simultaneously. 

Yes... That should work. Alternatives for (2) are to ground one of the 
enable lines and connect the other one to the enable line from your 
address decoder. Watch out, though, that many EPROMs take a lot more 
supply current when enavbled (CE/ low) than when they're disabled, so if 
the PSU is marginal-ish, you might have problems if the EPROM is held 
enabled all the time. Linking OE/ and CE/ together avoids this, but note 
that the CE/ assetyed - to - data - valid time is often longer than the 
OE/ asserted - to - data - vaolid time, which might be a problem in 
timing-critical systems.

I've done this sort of thing many times, normally to replace a masked ROM 
with an EPROM containing a modified version of the firmware. What I 
normally do is put a turned pin socket in place of the EPROM on the PCB. 
Then make up a little daughterboard using stripboard, another socket, and 
some turned pin header pins. Cut and jumper as appropriate. Of course you 
put the socket sltightly offset from the pins (but not totally outside 
them) so as to be able to use the stripboard tracks for the signals you 
don't want to move around.

This way I can easily go back to the original set-up if I want to.

-tony