Amiga 2000 internal serial header

[Castellen]

Anyway to check if 1488/89 are healthy or not?

For a start make sure that the +12V and -12V supplies are valid, they won't work otherwise.

You should measure approx. +12V on pin 14 of both ICs.  And about -12V on 1488 pin 1, and on 1489 pin 7.

A simple serial test is to get any terminal emulator (e.g. Term), set serial.device, unit 0, 9600 baud, no flow control and disable local echo.  Link pins 2 and 3 on the serial port to loop TX and RX data, then type anything on the keyboard.  If TX and RX data are working, you'll see the key presses echoed on the screen which will stop when the loop is removed.

If not, look on 1488 input pin 9 with an oscilloscope, you should see a 5Vp-p waveform with each key press.  Bit period = 100µSec.  That means the UART in Paula is generating serial data.

Then look at 1488 output pin 8, you should see a 24Vp-p waveform of the opposite polarity as the input.

If that's working, then check the same thing on the RX data path, with the serial port pin 2 & 3 loop in place.  i.e. 24Vp-p RS232 signal level on 1489 pin 1, and 5Vp-p TTL signal level on pin 3.

It helps to look at page 4 of the schematic to understand the circuit.  The 1488/1489 are simply a voltage level shifter and inverter.  Spares are cheap and readily available from the likes of Element14, Radio Spares, Digi-Key, Mouser, etc.

If there's valid RX serial data on the Paula RXD pin and it's not being decoded, it could be an interrupt problem.  But that's another conversation.

[Castellen]

Well, I swapped CIAs and now it turns on. What was happened?

Did you happen to accidentally short any pins on the 1488/1489 when you were measuring?  This can apply +/-12V on the 5V only CIA inputs which can damage the CIA.

Or it could be that old problem of poor contact in the IC sockets.  Removing then reinserting the IC will scrape oxidisation from the contact surfaces and can temporarily/permanently fix the contact problems.

Swap the CIAs back again, if the non-booting problem returns, then most likely you have a faulty CIA (8520).


Did you make any progress with the serial diagnostics?  The 1488/1489 should not normally get very warm.

At this point your repair options are:
1. Measure what's going on as described to work out where the problem is.

2. If you can't do this or don't have the test instruments, send it to someone who can do it for you.

3. Blindly replace parts until the problem is fixed.


I'm picking you're having trouble with the hardware diagnostics since you didn't report back with anything useful.  If you measured 24V across the 1488 (between pins 1 and 14) then the +/-12V supplies are OK.

Replacing the 1488/1489 could be a simple option as they're cheap and easy to get, and also the most likely to receive damage.  You'll need someone with good soldering experience to avoid making a mess of it.

If the part has been replaced as you say, and if someone has done a bad job, there could have been PCB damage or damaged tracks or solder shorts.  The damage may not be something you'll be able to easily see, but check carefully on the top and bottom side of the board.

It's much less likely to be a Paula/8364 fault.  But if you have an A500 spare, you can swap the 8364 and 8520s until you work out which ICs have problems or not.

Else just send the board to someone to repair.  A repair job this simple is typically a few minutes work.  There's myself in NZ and there are various others around the planet who can do the same.

[Castellen]

I replaced 1488 again but it didn't help. I may replace Paula next time.
According to this, 1488 is responsible for transmit not 1489, right?

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1.        Modem will not transmit. Most likely causes are:
o        1488 U38 (transmit) (U304 A2000)
o        8364 U3 Paula (U200 A2000)
o        8520 U8 Even CIA (U301 A2000)

2.        Modem or serial port will not receive:
o        1489 U39 (receive) (U305 A2000)
o        8364 U3 Paula (U200 A2000)
o        8520 U8 Even CIA (U301 A2000)

Yes, I'd agree with that high-level summary.  The 1488 is the RS232 line driver, so yes, this handles the serial TX and control line output.

Try another Paula/8364 if you have one.  It's not common for it to have UART problems though, but easy to swap with a known good part.

How do you know for sure that the serial TX is not working, how exactly are you testing this?  I would suggest not using Amiga Explorer for testing as it probably doesn't easily let you see what's going on in detail.

Plugging in a dial up modem is a better test.  When you send it AT[enter] it should respond "OK".

The best test is to connect to another computer with a known good null modem cable, then use a terminal emulator on each computer.  Set the same serial speed, disable flow control and see if keyboard presses are echoed on the opposite machine.