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Apple Lisa Port B stopped working.
Posted by: ivanthetolerable1 on 2026-09-16 17:15:54
Good day! I have a Lisa 2/5 that was given a new motherboard, repaired IO and CPU card and a 2mb upgrade board. Until 3 days ago the serial B was working and tested on LisaTerminal and LisaCom. It then stopped working. Serial A works no issue. I took the cage out and cleaned the connects again and still nothing. AI says maybe MC1488/MC1489 may be bad. They're not socketed too which is more fun.

Before I start making a space to desolder those chips I wanted to get a real human opinion.

Thoughts?
Posted by: stepleton on 2026-09-17 00:50:08
The problem could go all the way upstream to the SIO chip.

Is the problem bidirectional or just one way? That is, can you send but not receive or receive but not send? Note that the line driver IC that you mentioned is only used for sending; if it were broken, I might expect receiving to work still.

Do you have any measurement equipment? Serial TX and RX signals are usually pretty easy to spot on an oscilloscope. You can trace the data lines all the way back from the port (or out to the port) and see the exact spot a signal fails to pass through. That's probably your bad component, and you can try replacing it with some confidence.
Posted by: cruff on 2026-09-17 07:04:31
MC1488 is the line driver chip, MC1489 is the line receiver chip, so both directions could potentially be affected, but testing at those chips with an oscilloscope should narrow things down quickly.
Posted by: lisa2 on 2026-09-17 12:01:09
Given the this is a Lisa 2/5 with a new motherboard, and repaired IO and CPU cards, I am assuming the system was battery bombed and repaired. It’s normal to have battery damage in the area where the serial drivers are and unless all the traces are replaced with bodge wires, any corrosion that got under the solder mask will continue to grow. You should check all the traces in this area. Additionally because port B is differential, it uses 26LS30 and 26LS32 drivers, not 1488/89. Can you post a picture of your I/O board?
Posted by: ivanthetolerable1 on 2026-09-17 15:19:07
Hi everyone,

My only testing hardware is really a multimeter. Just for fun I reflowed the physical connector but that didn't help.

Yes the board was battery bombed but the Io wasn't affected that badly. Someone elsewhere mentioned connecting the port a and b together and running two lisa term programs and see if they'll talk. Does that sound right?
Posted by: stepleton on 2026-09-18 01:02:39
I wouldn't do it that way myself since it involves more of the serial subsystem of the Lisa, which is already behaving questionably.

Easiest to my mind is:

Have one terminal program on the Lisa set to 9600 BPS, 8/N/1, full duplex, no local echo, no hardware or software (XON/XOFF) flow control.
Have one terminal program on a modern computer set to the same settings.
Null-modem serial cable in between the Lisa and the modern computer.

Type on the Lisa --- does it appear on the modern computer?
Type on the modern computer --- does it appear on the Lisa?

If the answer is Yes to both: serial port is OK.
If the answer is No to both: I start to suspect the SIO.
If the answer is Yes/No or No/Yes: I start to suspect the driver/receiver IC for the direction that doesn't work.

Of course wiring issues or trace damage could also cause both kinds of trouble.
Posted by: ivanthetolerable1 on 2026-09-19 13:54:30
I wouldn't do it that way myself since it involves more of the serial subsystem of the Lisa, which is already behaving questionably.

Easiest to my mind is:

Have one terminal program on the Lisa set to 9600 BPS, 8/N/1, full duplex, no local echo, no hardware or software (XON/XOFF) flow control.
Have one terminal program on a modern computer set to the same settings.
IMG_0760.jpgNull-modem serial cable in between the Lisa and the modern computer.

Type on the Lisa --- does it appear on the modern computer?
Type on the modern computer --- does it appear on the Lisa?

If the answer is Yes to both: serial port is OK.
If the answer is No to both: I start to suspect the SIO.
If the answer is Yes/No or No/Yes: I start to suspect the driver/receiver IC for the direction that doesn't work.

Of course wiring issues or trace damage could also cause both kinds of trouIMG_0759.jpgble.
Thanks! I got everything in to connect it to my PC. What do I need to run on the Windows side? Also pics attached.
Posted by: stepleton on 2026-09-19 14:55:21
Hmm, good question; I'm not really a Windows person so it's tough for me to say for certain. PuTTY seems to be a popular choice and I think I remember using it in the past.
Posted by: ivanthetolerable1 on 2026-09-19 18:58:08
Hmm, good question; I'm not really a Windows person so it's tough for me to say for certain. PuTTY seems to be a popular choice and I think I remember using it in the past.
Progress! The Lisa can send to the pc but nothing on the pc side shows on the Lisa. Now what does that mean.... 1789869480465.jpeg
Posted by: stepleton on 2026-09-20 01:10:35
It does make the line driver IC start to seem like a more likely culprit than the SIO. The SIO handles both input and output, while there are separate driver and receiver ICs. So if there's a problem only in sending, it makes some sense to think that the issue might be limited to the hardware that's sending-specific.

But before we start drawing conclusions: since it's often easy to mess up serial port configuration, can you confirm that your setup is good by demonstrating that serial communications work in both directions with your cable connected to the Lisa's serial port A?
Posted by: ivanthetolerable1 on 2026-09-20 09:05:46
It does make the line driver IC start to seem like a more likely culprit than the SIO. The SIO handles both input and output, while there are separate driver and receiver ICs. So if there's a problem only in sending, it makes some sense to think that the issue might be limited to the hardware that's sending-specific.

But before we start drawing conclusions: since it's often easy to mess up serial port configuration, can you confirm that your setup is good by demonstrating that serial communications work in both directions with your cable connected to the Lisa's serial port A?
Serial Port A confirmed to be able to send and receive data from the PC.
Posted by: stepleton on 2026-09-20 09:38:58
Good. At this stage, if you are unable to get your hands on an oscilloscope so that you can see the signals moving around on the I/O board, it might be time to think about replacing the receiver IC. Just so I understand correctly: what you type on the Lisa shows up on the PC, but what you type on the PC does NOT show up on the Lisa. If that's right, then U11E, the 26LS32 line receiver IC, is a primary suspect.

Let's just check a few things first to eliminate the possibility of connections being a problem. Using your multimeter's continuity checker (or ohmmeter), you should find that all of these things are connected (they should have less than an ohm or two of resistance between them):

1. U11E pin 3 and U9C pin 27 (the wire between the receiver and the SIO ICs)
2. U11E pin 1 and motherboard IC9/RP2 pin 14 (the wire between the receiver and the serial port resistor pack)
3. Motherboard RP2 pin 7 and serial port pin 3 (the wire between the serial port resistor pack and the serial port)

Naturally you will need to measure 2 with the I/O board plugged into the motherboard.

4. Measure between motherboard RP2 pins 14 and 7 --- there should be some resistance but not loads of it. Report it here.

Then with a voltmeter, with the Lisa running but NO cable plugged into the serial port (you will need to defeat the power supply panel interlock switch for your Lisa to run):

5. Measure the voltage at U11E pin 1; we expect +5V.
6. Measure the voltage at U11E pin 2: we expect 0V.

If all of these measurements look reasonable, then (unless you can get your hands on an oscilloscope), I think it might be worth replacing U11E and seeing if things improve.
Posted by: ivanthetolerable1 on 2026-09-20 11:04:54
UPDATE:

I stole an 26LS32 from a Macintosh Plus 1mb and tossed it into the Lisa and Serial B works! Thank you so much everyone for the help.
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