Common A1200/4000 deaths, CIA chip

[amigaksi]

On the A500, A2000, A3000, and A1200, the signals I was referring to are bi-directional already since they are not going through the 74125A like they are in an A4000.  On the A1000, they are going through the 7407 which makes them uni-directional.  Regardless, they should not affect the normal functionality of a floppy drive since the floppy drive only uses them for input (output on the Amiga end).  However, in reference to the floppy simulator:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320148543065

I wanted to support normal floppy functionality as well as bi-directional parallel communications for faster transfers once the system boots so I was trying to trace down why I was unable to do bi-directional parallel transfers on the A4000 and A1000.  

One thing I did notice is that floppy failure is not necessarily the CIA fault since the 7407 or 74125A or some other supplementary chip could be faulty and the CIA chips could be just fine.

[amigaksi]

>Keep in mind that faults elsewhere in the Amiga could cause >the same problems, such as broken traces, shorts or faulty >components between the CIA and the floppy,parallel and serial >ports.

Right.  And also the other components can cause failure of the CIA chips.  I have seen a bad 7408 chip pop the top off a 8520 chip in an A1000 (and I could see the transistor grid behind the black cover).  [The 7408 is the invertor chip with various timing signals.]

Anyway, you can modify the Amiga 1000 7407 to make the CIA bidirectional just like it is on the Amiga 500.  The way I did it was to disconnect the +5V (VCC) pin.  Connect the A and Y signals together for the DIR, SIDE, and STEP signals.  This effectively makes these signals bypass the 7407.  The floppy dirve and floppy simulation still work and the signals can now be used bi-directionally.

Addendum: The above proves also that if you have a faulty 7407 chip on the Amiga 1000, you don't have to replace it.  You can just bypass it by making the signals go through directly from the 8520 to the floppy connectors using modifications similar to the ones I made.

[webhead]

hello can somebody tell me where the cia chip or chips are on a 1200 please as my poor 1200 has died i only get a white screen and no kickstart put 3.1 roms in but does the same i get keyboard lights and numlock stays on .

just wondered if it could be cia chip?

[amigaksi]

Disconnect the keyboard and see if it boots.  I have an Amiga 500 with the keyboard where the RESET is jammed so you just get a white screen as if someone continuously held down the reset key.  If you are technically inclined and find it's the keyboard, you could use the simulated keyboard with it by modifying the A500 or A2000 keyboard cable available here:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320148538163

It's helpful for diagnosing even if you decide not to always use it.

[Merc]

At the risk of hijacking the thread, has anyone ever tried using an A500 style CIA chip to replace one on an A1200?  

Years ago I got hold of a couple of A1200 boards that had had their CIAs removed, and I found the pinouts and soldered up the chips from an A500 onto the 1200 board.

It looked like it was going to boot, but then the power LED blinked some kind of error code and it didn't go any further...  Either the boards had some other problem (likely) or something was wrong with the transplant...

[amigaksi]

...
>Years ago I got hold of a couple of A1200 boards that had had >their CIAs removed, and I found the pinouts and soldered up >the chips from an A500 onto the 1200 board.
>
>It looked like it was going to boot, but then the power LED >blinked some kind of error code and it didn't go any >further... Either the boards had some other problem (likely) >or something was wrong with the transplant...

I never tried that but there is some difference in the frequency tolerance between the Paula/8520 (CIA) combo floppy controller on the Amiga 1200 verses the Amiga 500.  I was able to run a floppy simulator using only 950KB/second parallel port on the Amiga 500 and Amiga 1000/2000/3000, but on the Amiga 1200 I needed a parallel port on the PC that was much closer to 1 Megabytes/second.