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
|
