#No serial response
8 messages in this thread
I bought a used A500 recently and I don't seem to be getting any response out
of the serial port. Are there any diagnostic tests I can run to determine just
what the problem is? When using Platinum Online the software tries to dial,
but there's no modem activity. Even typing AT commands gets no response. I
know it's not the modem or cable as they work fine when hooked to another
computer.
Any suggestions?
– Barry Jarrett
careening around CIS on my Amiga!
Barry,
It could be one of several things…and I know of no diagnostic test
that will specifically check out the serial port…but there is one thing you
may wish to do:
Presuming you know what you are doing diging around the innards of a
computer (if you don't…then ignore this!)…open it up and swap the 2 8520
chips. Now try the serial port. If it works, you've got a bad 8520…if not,
one of the other chips…either PAULA or the driver chip…is bad.
Don
Don,
I'll give the chip swap a try…. (crossed fingers on this one!)
–Barry
>>either PAULA or the driver chip<<
If somebody has a dead serial port, it is most likely the driver chips(s).
The UART itself is inside of Paula(along with other things).
The good news is the driver chips can be had at radio shack for a couple of
dollars, bad news is you need to unsolder the old one(s).
Here is how to tell, connect a modem to the port, start terminal program. now
watch the lights on the modem for response as you type on the keyboard. I type
'ATZ' and see what it does. If it does nothing, have someone call the line and
see if you see 'ring' on the screen.
There are 2 chips, Quad line driver, and Quad line receiver.
You should be able to figure it out from here….
I have made this repair in the past, put in sockets in case it happens again.
I don't believe the CIA chips are involved with the serial port at all, but I
would have to double check the prints.
From my experience, the receiver chip seems to be the one to go.
Hope this helps.
-Peter
Peter,
Thanks so much for the tips… If the driver chips are readily available,
then I can get an electrical diagnostic technician friend of mine to do the
dirty work (a soldering gun in my hands is a dangerous thing!).
I did the CIA swap which Don suggested, to no effect. I was able to swap out
the Paula chip with that of another A500 I have, to no effect. To make sure it
just wasn't some freaky dislike that my modem might have had for A500s, I
hooked up the 'good' A500 (after putting the original Paula back) and it worked
fine. Soooooo.. it looks like the driver. I'll have 'em both replaced….
Thanks again. I'm trying to make this new A500 into a portable for a trip I'm
taking in a week, so the serial port is of utmost importance.
– Barry Jarrett
careening around CIS on my Amiga!
Don,
I did the 8250 chip swap… no luck… 🙁
I checked the schematics in the back of the A500 book, and it I'm assuming your
reference to the PAULA chip is about the RxD and TxD leads which run to the two
chips, U38 (1488) & U39 (1489), which themselves lead to the serial port. I
know the Paula chip is in a socket, but I neglected to check if U38 and U39 are
socket or soldered….. I guess I could pull the Paula out of the A500 at
work to see if it's that….
If you have any other suggestions (apart for sending it to a repair facility),
please let me know.
– Barry Jarrett
careening around CIS on my Amiga!
Barry,
The 1488 and 1489 are the line driver for the receive and transmit
lines. If they're bad, you get nothing in and/or out (depending on which may
be bad). They are connected to the PAULA chip…which actually generates the
signal.
Don
and they are soldered in – sockets and replacements at any Radio Shack.