[Gmecm] MEMCAL programming

[bcroe at juno.com]

There were some EPROM chips made with a cheap (no window) 
plastic case, instead of the expensive ceramic and glass.  They 
could be programmed the usual way.  But with no window for 
erase, they could only be programmed once.  Bruce Roe

On Fri, 25 Nov 2005 09:46:36 +1100 "Jon Dods" <jdods at bigpond.net.au>
writes:
> Hi,
> 
> Most likely the chips in the Memcals without the window in the top 
> are PROM's not EPROM's or EEPROM's.
> Once PROM's are programmed they cannot be erased.
> 
> There are a number of PROM's and EPROM's which are functionally and 
> pin for pin the same once programmed.
> I'm not aware of any EEPROM's that also fall into this category.
> The only thing you need to be aware of is the access speed of the 
> chip, this is usually stated on the end of the part number.
> Eg; 27C256-200, 200 being the speed.  A replacement chip must have 
> an equal or faster access time (lower number).
> 
> There is also a slim chance that these chips without windows are 
> PLA's, (Programmable Logic Array's).
> These can be programmed in a fashion where they will emulate the 
> code in an EPROM but cannot be read back, so it stops people 
> 'stealing' the code in after market Memcals.
> 
> PROM - Programmable Read Only Memory
> EPROM - Erasable Programmable Read Only Memory
> EEPROM - Electrically Erasable Programmable Read Only Memory