@davesmeghead those are the exact two cables you listed from Amazon. I was trying to put the Amazon links but as my account is new, I was prevented from doing so.
@pocket Thank you for the references. These are cables that I had previously researched long ago. I had tried them and they worked with my RPi.
I had used / tested them with two pieces of software and they seemed to work fine.
One was the VisonicDriver for Windows: Home Automation Domotica Forum Europe, Bwired Forum • View topic - Visonic Powermax Interface (Windows) application
The other was using a Linux daemon found at: GitHub - viknet365/pmaxd: daemon for communication between linux and a visonic powermax alarm
The last one was described at Home Automation Domotica Forum Europe, Bwired Forum • View topic - Powerlink serial pinout, linux daemon and web interface and then Step by step guide: Visonic Powermax Pro integration - Domoticz
I don’t remember having to install any special software on the RPi to make the serial port visible. I did have to install drivers to make the Startech work in Windows. The above two pieces of software worked without problems. FYI, I also used the Visonic Powermaster Remote Programmer successfully, which is a Windows program that can be used to program the panel.
The Powermax+ has a location for connection to an external Powerlink module or an external GSM modem. There is a small adapter that is plugged in and provides a female RJ45 port that can be plugged into.
I’ll take pictures and post.