(3, 4) OK. sorry, but I expecting stuff to go iteratively somewhat. I worked with RS232 since the mid 1970's.
I had the benefit of an RS232 Breakout box. Something like this with the adapters makes it CAKE! The voltmeter method makes it MUCH harder.
When you connect a receive to a receive - nothing bad happens except it doesn't work. Note here:
https://www.ti.com/lit/ds/symlink/sn75188.pdf is a typical driver chip. Note a 300 ohm resistor and diodes to the + and - supply. This is basic protection. It would not like all of it's output shorted to ground, but to each other is OK.
You have to find ground. Inspection or an ohmmeter to ground. Usually it's a solid connection to chassis ground and power supply common. Re-check with the diodes mode to make sure there are no diodes in your ground connection. Even if you don't initially find ground, you can measure the voltages with respect to chassis.
You also need to know if the device is a DTE (Terminal) or a DCE (Modem). If you know ahead of time great.
Here:
https://en.wikipedia.org/wiki/RS-232 is the RS232 pinout, but not the IBM PC version. Note that you have voltages for assertion and a device side.
You have genders and crossover cables, so it makes it more complicated.
If you can get a breakout box, it really makes it easier.
Because they have SIDES, and it used DCE and DTE then you can look at these signals as "the device has power". Inputs won;t have any voltages on them. RS232 chips used to handle +-25 V; modern devices use +-12-15 V
If you can tell the plotter to ignore RTS/CTS all the better.
get ground
The next easy one is DTR/DSR or CTS/RTS. Some devices used to really mix up the signals back in the day.
If you only had 5 wires, you have Rxd, Txd, ground and normally RTS/CTS, but remember they are all relative to a DTE/DCE.
RTS/CTS and one of Rxd/Txd are generally not strictly needed. What I mean is, RTS/CTS is hardware flow control and Rxd/Txd are used for communication back to the Plotter driver. So, you can get a small file to plot with only 2 wires. Large files might plot somewhat. Without the other Rx/Tx there is no error communication back to the driver.
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Ideally, you should have the standard gender 9 pin port on the PC and the reverse gender on the plotter and a straight-thru cable. So, this information,ground, the five signals used and the voltage readings, we have enough info to start.
So, except for a single short to ground, any pin can touch any pin on the PC or Plotter side without damage. You only have one ground pin.
As I understand it, only 5 wires are connected. Knowing which pins have remnants would help too. All good information.
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Here
https://www.electro-tech-online.com/threads/serial-rs232-cable-help.147785/ is a VERY SIMILAR thread.