22 Sep
2011
22 Sep
'11
10:38 p.m.
I had the same ideas yesterday. Thankfully the ROM is
socketed, so I read
You were lucky. In one device I was working on recently, the microcode
ROMS were not only soldered directly to the PCB, there wasn't enough
headroom to put 2 of them in sockets. 3 of them went back in noraml
sockets, but the other 2 would have bnaged into the mains transformer.
Fortuantely, in this case the PCB holes were 1mm diameter so I could fit
some of those zero-height socket pins that go through the board.
it out and found out its function and its fault. The
function is more than
simple... it just inverts the inputs, i.e. output M0 is the inverse of the
I am sprised they used a ROM for that. Is the enable input used -- is there
some other open-colelctor output tied to the outputs of this ROM? If not,
you could do it i na copuple of TTL pacakges I think.
address line A0. The sixth output M5 goes active (low)
when all inputs are
low (zero difference detect). The fault was the A3 input. Therefore the
addresses x8-xF were mirrored at x0-x7, and the drive always assumed a
Right,... I'ev had several ICs fail with TTL inputs that appear to be
stuck high (as here) and the pin tests as a total open-circuit to the
power lines. I wonder if the failure is actually the bond-out wire
conencting the pin to the die, not the circuitry on the die itself. Not
that it really maters, I cna't rebond an IC here (yet!)
Do you know that it simply inverts the signals, or could it do soemthing
differnt for small cylinder offsets?
-tony