Serial line questions on Keithley 2700 display

[yxngxr_11]

I'm trying to fix an old Keithley 2700 I got from a school lab. The screen is mostly dead and only a few parts light up. Hoping to replace it with a small OLED screen using a microcontroller. I saw that the display board uses an 8051 chip to control the VFD with shift registers. I checked Port 2, but one pin (P2.1, maybe called SEGDATA_I?) doesn't seem to work.

The rest of the board still works fine, including the buttons. I'm wondering if I can read the data from the serial line between the main board and the display instead. Has anyone figured out how that works on Keithley or HP units like this? The manual says the speed is 192k baud with 1 stop bit. Just want to know if that's correct.

 

[General_Ebdahl]

I haven't cracked a 2700 myself, but I doubt you'll get lucky with one clean pin feed. Best move is sit on the inter-board ribbon with a logic probe and watch what happens when the thing updates the screen. That'll tell you more in an hour than digging through Keithley docs for a week.

 

[francsxz39]

I'm trying to fix an old Keithley 2700 I got from a school lab. The screen is mostly dead and only a few parts light up. Hoping to replace it with a small OLED screen using a microcontroller. I saw that the display board uses an 8051 chip to control the VFD with shift registers. I checked Port 2, but one pin (P2.1, maybe called SEGDATA_I?) doesn't seem to work.

The rest of the board still works fine, including the buttons. I'm wondering if I can read the data from the serial line between the main board and the display instead. Has anyone figured out how that works on Keithley or HP units like this? The manual says the speed is 192k baud with 1 stop bit. Just want to know if that's correct.

Cool that you're fixing up an old lab gear! That P2.1 issue is a bummer, but tapping into the serial communication between the main board and the display board is a smart workaround. The 19200 baud, 8 data bits, 1 stop bit, no parity (19.2k/8/N/1) setting is very likely correct for that era of Keithley equipment. They tend to stick to standard communication parameters. However, the protocol is almost certainly proprietary, so it won't be standard SCPI or ASCII. You'll need to sniff the data with an oscilloscope or logic analyzer to reverse-engineer the custom byte structure that controls the VFD segments.

 

[Alan]

If SEGDATA_I isn't working, check the shift register's latch enable or clock line. Even if there's no data, the timing signals could show how the segments are updated.