woot: ROM tradgedy revisited

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
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
- woot --- ROM copied and image working

- 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 supports 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

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,

Re: woot: ROM tradgedy revisited
robb wrote:
Quoted text here. Click to load it

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  

We've slightly trimmed the long signature. Click to see the full one.
Re: woot: ROM tradgedy revisited
On Thu, 10 Jan 2008 23:58:31 +0000, Ian Malcolm

Quoted text here. Click to load it

Ian, do you have ANY evidence to back that claim re sunlight.  AFAICT it is
urban legend, but tests we conducted - in a far sunnier climate than yours ;-) -
revealed not a single bit had changed in 2764's after a month in the weather.


32S 116E

Re: woot: ROM tradgedy revisited

Quoted text here. Click to load it
;-) -
Quoted text here. Click to load it

Should just be a matter of time, given photons as quanta. I couldn't do it  
here, light not strong enough, but a less-than-annihilating focus with a  
magnifier speeded it up enough to prove I could erase and re-write, so the  
question is not whether unfiltered sunlight can do it, it's how long does it  
take for a given strength.


51N 238W

Re: woot: ROM tradgedy revisited
Quoted text here. Click to load it

It's not just the quantity of photons, but also their frequency, which is
why x-rays won't erase EPROMs.  (Sufficient x-ray dose will damage the
part, though.)

The spec is 2537 angstroms, but I think that was selected by Intel because
it was the nominal wavelength from germicidal bulbs.  The parts are sensitive
to a range of wavelengths, but I've never seen a response curve.

Natural sunlight doesn't have significant content at or near 2537 angstroms,
or we'd all be blind.

Re: woot: ROM tradgedy revisited

Quoted text here. Click to load it

Irrelevant. It's there, or partial focussing would not be all it takes to get
enough to do it within the few minutes I did it in. So it is just a matter of
time. I know what spectral band it needs, that wasn't the issue here. I'm not
attempting practicality, just saying to those who say it can't happen, that
it can, it's just a matter of how long it would take in unmodifed direct
sunlight. (Too long).

Re: woot: ROM tradgedy revisited

Quoted text here. Click to load it
I was involved with some tests on EEPROMs, many years ago (back in the
days when the 2716, was 'new' technology).
We had 32 test chips, and put them on the lab windowsill for six
months, without a single bit error.
However we also put another set out in direct sunlight. On these, two
had bit errors after 3 months, and at the end of the test, five showed
The difference,the  glass in the lab windows.
Ordinary glass is quite opaque to the required frequency (253.7nm), so
it is unlikely that chips will get erased in normal room lighting, or
from daylight through windows, but if the unit is outdorrs, the Sun,
does have the energy to cause problems over a long period.

Best Wishes

Re: woot: ROM tradgedy revisited

Quoted text here. Click to load it

Sorry to but in, but I had an 8748( onboard eprom) project back in the mid
70's..... worked fine on the bench, wouldn't run in the box..... I had to
put a 6.3V bulb, running at 5V over the widow to make the dam thing work.
Neither I nor Intel ever figured out why ....... strange but true!

Re: woot: ROM tradgedy revisited

Quoted text here. Click to load it
That is 'fun'. :-)
If you remember that all the exposed semiconductor junctions, _do_
have the ability to behave as photodiodes, I'd guess that the light
was fractionally altering the bias current somewhere in the die. A
very 'unusual' fault!...

Best Wishes

Re: woot: ROM tradgedy revisited

Quoted text here. Click to load it

Agreed, nice find. Probably is photodiode effect, the near IR that silicon is
most sensitive to is strong in a small incandescent, almost ideally matched.

Site Timeline