General Category > Lisa Troubleshooting and Repair
Uh oh... Did I kill a serial port?
sigma7:
--- Quote from: jamesdenton on September 17, 2024, 09:08:12 am ---I've been meaning to make cases for these things but...
--- End quote ---
If you're not in a hurry, I've done some simple designs in FreeCAD for 3D printing, so might be able to help.
In the meantime, fastening some thick paper or cardboard over vulnerable parts of the exposed circuitry helps me prevent accidents with prototypes.
ried:
--- Quote from: sigma7 on September 17, 2024, 01:34:50 pm ---On the motherboard, there is an interference filter for each serial port which can be damaged. The one for Serial B is RP2.
If you measure the resistance across the part between most pairs of pins that are physically opposed, you should see about 50 ohms. ie. pin 2 to pin 19, pin 3 to pin 18, etc. ... except for the pairs at the ends (pins 1,20 and 10,11), which are directly connected. If you have an open circuit between any pairs then that may be the point of failure.
Another possibility is that a connector contact or circuit board trace was damaged by the high current. I think only the ground pin (pin 7) would be at risk from a 5V source. You should measure close to 0 ohms between pin 7 and the metal chassis.
--- End quote ---
I measured ~51-53 ohms of resistance across all of the interference filter pin pairs, with no resistance at 1,20 and 10,11. So that seems to be okay.
On the DB-25 ports themselves, I also measured 0.1 ohms of resistance between pin 7 and the metal chassis, so that also checks out (on both Serial A and B). Out of curiosity I measured resistance between the other pins and the metal chassis (I hope that's okay?), hoping to discern any difference at all between measurements of Serial A and Serial B. On pin 10 4 of Serial B, there are 12.68 k ohms of resistance between it and the metal chassis, but that is not present on pin 10 of Serial A. Might that be a clue? Or is that the way Serial B was designed?
Hopefully all my poking and prodding isn't making anything worse.
sigma7:
--- Quote from: ried on September 18, 2024, 01:40:07 am ---On pin 10 of Serial B, there are 12.68 k ohms of resistance between it and the metal chassis, but that is not present on pin 10 of Serial A. Might that be a clue? Or is that the way Serial B was designed?
Hopefully all my poking and prodding isn't making anything worse.
--- End quote ---
If you've got the power unplugged and staying away from the video circuitry and power supply, poking around with an ohmmeter is unlikely to damage anything in a Lisa.
Pin 10 of the RS-232 ports isn't supposed to be connected to anything, so double-check that you've got the pin numbering scheme correct...
Looking at the rear of the Lisa, you see the receptacle side of the female DB-25 connector, so the pins are like this:
--- Code: ---\ 13 12 11 10 9 8 7 6 5 4 3 2 1 /
\ /
\ 25 24 23 22 21 20 19 18 17 16 15 14 /
--- End code ---
A minimal RS-232 connection requires 3 wires/pins
Pin 2 - Transmit data from Lisa
Pin 3 - Receive data to Lisa
Pin 7 - Ground
Sometimes more wires are used for handshaking and modem control.
Assuming you have a 2/10 I/O Board plugged into a 2/10 Motherboard, you should be able to measure low resistance from the I/O Board to Serial B as follows:
--- Code: ---Serial B
DB 25 - to - I/O Board
Pin 2 - to - U10E-15
Pin 3 - to - U11E-1
--- End code ---
If you turn the power on, the voltage at Serial B pin 2 should go to -5V or so when nothing is being transmitted.
ried:
--- Quote from: sigma7 on September 18, 2024, 02:50:38 am ---If you've got the power unplugged and staying away from the video circuitry and power supply, poking around with an ohmmeter is unlikely to damage anything in a Lisa.
--- End quote ---
Excellent. Thank you for confirming.
--- Quote from: sigma7 on September 18, 2024, 02:50:38 am ---Pin 10 of the RS-232 ports isn't supposed to be connected to anything, so double-check that you've got the pin numbering scheme correct...
Looking at the rear of the Lisa, you see the receptacle side of the female DB-25 connector, so the pins are like this:
--- Code: ---\ 13 12 11 10 9 8 7 6 5 4 3 2 1 /
\ /
\ 25 24 23 22 21 20 19 18 17 16 15 14 /
--- End code ---
--- End quote ---
Thank you for that numbered pin layout. Here is my corrected statement: "On pin 4 of Serial B, there are 12.68 k ohms of resistance between it and the metal chassis, but that is not present on pin 10 of Serial A. Might that be a clue? Or is that the way Serial B was designed?"
sigma7:
--- Quote from: ried on September 18, 2024, 03:06:43 am ---On pin 4 of Serial B, there are 12.68 k ohms of resistance between it and the metal chassis, but that is not present on pin 10 of Serial A. Might that be a clue? Or is that the way Serial B was designed?"
--- End quote ---
Pin 4 of Serial B has a 10K resistor to +5V on the Motherboard, so measuring about 12k to ground is reasonable. Pin 4 of Serial A is driven by a line driver on the I/O Board under control of the SCC, so it will appear different.
So, no clue there... keep looking!
edit: The 10k resistor to +5 on Pin 4 of Serial B is one of the differences between the Lisa 2/10 Motherboard and the 1/5 Motherboards. The Apple 1/5 Motherboards do not have this resistor, so depending on the wiring of the serial cable, a serial device expecting RTS to be high may not work on Serial B of a 1/5 but still work on Serial A. The Sapient variety of the 1/5 Motherboard does included this resistor.
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