robb wrote:
> woot and and hardy THANKS ! to all the help,
>
> more small problems with questions at the bottom if you want to
> skip the story ....
>
> short version:
> ----------------
> - robb repairs non-problem on working ('85) micro-board with 32v
> rampage on the 5v rail (32v rails meets 5v rail) :( bad joke,
> nothing repaired, everything broken :(
> - ** lots of help from "sci.electronics.*"
> - many IC casualties including the MCU (SAB 8031) and ROM (NEC
> D23256AC) **BIG** ROM problem
> - ** lots of help from "sci.elec.*
> - much searching and patience then access to good ROM *but* many
> problems reading
> - ** lots of help from "sci,elec.* and "rec.games.pinball" and
> "alt.microcontrollers.8bit"
> - woot --- ROM copied and image working
>
>
> Details:
> -------------
> - ROM mistakenly treated as 27c256 variant. so, bad reads and no
> pin play helps
> - ROM turns out to be maskable, address latching. so need a slow
> or latching alogortithm
> - helpful advice says try 87c257
> - The programmer sup****ts it and it worked --- good ROM read
> - Programming , no joy with 27c256 variant ROMs as a new ROM host
> even though in theory it seems they should work
> - new 87c257 arrive and work
> **BUT**
>
> Problem:
> -----------
> On the 87c257 (Pin 1) is (~AS, addr strobe) is the latching
> signal. However, the (D23256) used falling edge of (OE) same as
> (G) as the address latch ??
>
> so 87c257 only works if i lift (Pin 1) out of socket and jumper
> to a suitable/safe address latching signal
>
> Question (s):
> - both (OE) and (CE) work as surogate latch but which is better
> choice ?
> - what is the best/proper way to solve this problem or do this
> jumpering ?? just solder a jumper wire across the pins ? should i
> use resistor, diode, other ?
> - how to deal with dangling (Pin 1) fold it up, clip it, etc
> - Is it possible to get the 27c256 ROM without adding latching
> mechanism around them
> any suggestions on what pieces i would need to build a
> latching mechanism
>
> thanks again for any helpful advice,
> robb
Always consider the possible need for youself or another to be able to
read the new rom in a programmer in several years time. Therefore do
*NOT* butcher the pins more than you have to. Clipping the pin will
make it more difficult to read in the future, so dont if you can get
enough clearance for safety by bending it out a bit (not dead flat, it
might break off). IIRC your board had /ALE going to pin 20 (/CS or chip
select) of the socket and PSEN to pin 22 (/OE, outpout enable). A
carefull inspection of the SGS Thomson M87C257 data sheet indicates that
it should be acceptable to tie pin 1 (bent out) to pin 20 driving both
the chip enabe and the address latch from the same /ALE signal. Thats
*ONE* wire soldered diagonally accross the top of the ROM. I've seen
worse in production equipment!
No resistors diodes etc. required. The *ONLY* time you should use a
resistor for a mod like this is if you need to tie an input to +5V
supply, then you should use a series resistor typically 1K to prevent
any risk of latchup on a supply spike.
Dont forget a bit of aluminium foil tape over the window in the EPROM.
It on1y takes a few weeks of direct sunlight to wipe an EPROM and
ambient liting can cause the EPROM to malfunction (But not actually
erase it). Some people would prefer a black paper label. Pale colours
and PVC tape can let too much light through for *long* term reliability
At this point, with pin 1 tied to pin 20, assuming it works OK, leave a
note in an envelope taped inside the cabinet for any future repair
describing the modification and for ****s sake put it back together
before the cat knocks it off the bench or anything else goes wrong :-)
As to making the ordinary 27Cxx series roms work with an 8051
equivalent, You would need that address latch, all the circuits I have
seen use an external TTL octal latch to demultiplex the Adrdress bus.
I'm not saying it could *NEVER* work without a latch as an old enough
slow enough EPROM *might* have the data valid long enough from a
transition on the address bus, but if you replaced with a different
brand of 8051 or EPROM or even changed the temperature or supply voltage
10%, I would expect it to stop working! *NOT* exactly a sane and
reliable design :-)
If the original manufacturer is no more, please consider putting the Hex
file for the ROM, with full make model and description to accompany it
on the net somewhere suitable, *somebody* may need it sometime in the
future.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@[EMAIL PROTECTED]
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