Date : Wed, 15 Apr 2015 00:48:59 +0100 (WET-DST)
From : bbcmicro@... (Peter Coghlan)
Subject: EPROM versus ROM for BBC/B OS1.2
>
>> Wrong type of EPROM?
> (ST)M27128A (should be 16K)
>
I found a data sheet for the M27128A-2F1 by ST Microelectronics on the web
and I had a look through it. As far as I can see, it should be suitable. The
M27128A-2F1 variant is 200ns which is more than fast enough. Hopefully it is
not too fast! (The M27128A-F1 is 250ns - that should work too and the
M27128-3F1 is 300ns which might also work.)
>
>> EPROM not correctly programmed?
> Program verified ok with programmer.
>
Can you read the original OS ROM in your EPROM programmer (without trying to
program it and without applying the programming voltage) and compare the image
with the image you are programming into the EPROM? This might show up any
problems in transferring the image from wherever it is coming from, to the
EPROM programmer.
>> Some incompatibility between the hardware and OS1.2?
> Fitted with Data-Center, so board was modified so that 2 ROM sockets are
> seen as 3 each and another for 1 RAM
> if I remember correctly.
>
I'm not familiar with that item - I don't know if it would have any bearing
on the problem. I guess if it works with OS1.2 in a ROM, it should work
equally well with OS1.2 in an EPROM. If it has connections to jumpers on the
beeb circuit board, it may be suspect.
>
> Sorry, my fault, did not send enough information as I didn't expect I would
> get so much interest & help!... thanks.
>
Don't worry - there are teams of BBC micro people lying in wait for interesting
problems to solve :-)
>
> Just experimenting, tried putting my name instead of eg: "BBC Computer".
> The machine works fine with original ROM and also tried exact OS1.2 image in
> EPROM to make sure but no go.
> Machine beeps continuously and shows a blank screen when powered up.
> Blank.. no screen.
>
Looks like the EPROM is not being accessed correctly or the CPU is not managing
to read the startup code from it. I suspect the behaviour would be the same
with no OS plugged in.
I don't have a data sheet for the ROM but from looking at the model B circuit
diagram, it seems very likely that the only differences between the ROM and an
EPROM could be at pins 1 and 27. The EPROM data sheet says that these should
both be high when reading from the device but is not very explicit on what will
happen if they are not high. The circuit diagram says they are connected to
A14 and A15 (which are both high any time OS locations are being accessed).
Perhaps these two pins are not connected to anything on the ROM? If so and
there was a fault with connections to either of those two pins on the circuit
board, a problem might show up when an EPROM is used but not a ROM. It might
be worth verifying that these two pins are correctly connected to A14 and A15.
>
>> Does this help? :
>> "When replacing this with a Series 1 Operating System ROM it should
>> be noted that the carrier board has rather large diameter pins, which
>> sometimes damage the IC socket into which the board was fitted."
>
>> Take from:
>> http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw_03.htm
>
>
> This is not the case... large diameter pins, no... it's a normal EPROM with
> normal pins.
>
I wouldn't rule out problems with the socket. They are not very good sockets
and they can end up making good contact with pins on one chip which are bent
one way and poor contact with pins on another chip which are bent slightly
differently.
Regards,
Peter Coghlan.