You made a couple of measurements that to me indicate a NULL modem cable.
I really want to warn you how the pins go. Pay attention to the numbering.
The top one, is probably the most likely to work: **broken link removed**
If you know anything about the plotter, such as:
DTE/DCE
Software/hardware handshaking - what type DTR/DSR or CTS/RTS
The attempt to look at the unconnected voltages with respect to SG at the pins is to identify the port as a DTE or DCE.
If 5 & 3 or 5 &2 both had voltages on them at the computer and the plotter a straight through cable would not work.
Voltages at DTE or DCE would tell you in a real situation which type of device it is.
To give you an idea of how things work:
1. DTE/DCE tell each other that the device has power
2. RTS/CTS are hardware handshaking signals
3. TXD/RXD are data
4. SG is signal ground
Only 3 signals are required SG, TXD and RXD if software handshaking is employed.
I've seen real messes with 1 and 2. Like reversal of roles and bad assignments.
There are two other signals.
RI - ring indicator and CD - Carrier detect
RI is a signal telling you the phone is ringing
CD is a signal that the modem has carrier.
One of the RTS/CTS signals says - Answer the phone
SO it like ring(RI), answer phone (RTS?). wait for Carrier(CD), CTS
So, there were ways to disable "modem control" in software or make a few jumper connections to disable modem control.
So DTR/DSR put the modem and or computer offline.
DTE - Data terminal Equipment and DCE Data Carrier Equipment (The phone companies modem)
This was all implemented in a 25 pin connector. IBM came along and made a 9 pin connector. the 25 pin connector was used for the printer instead of the Centronics connector.
To really make this make sense, I'd have to verify the directions of the signals.
That's why I really wanted you do do some measuring from pin 5.
The IBM male side should be well known as to what it is. I still need a cheat sheet.