(This answer is based on the MAX2312 capabilities, so only relevant for machines using it, which are, as per Question edit, all Psion 3C and later)
I have read that the serial port was powered by a Maxim MAX3212 chip, which runs at around 3v. However, it isn't clear whether Tx and Rx run at Rs232 12V, or at 5V or even at 3.3V.
Erm, AFAICT its datasheet does cover all mentioned parts.
Regarding voltage on (true) RS232, +/-12V is only the most common nominal one. Acceptable voltage is defined as +/-3V to +/-15V (*1). So anything above 3V in either direction is RS232.
The chip is intended to convert positive logic level from an UART signals to RS232, thus enabling operation with most RS232 devices, as well as protecting the UART side from otherwise harmful voltages.
- It operates from a 3 Volt supply
- It converts 3V logic signals on the UART side to positive/negative voltage on the RS232 side and vice versa.
- It provides +/- 5V as output (that's TX)
- Under certain circumstances output can be up to +/- 10V
- It can handle up to +/- 25V input (that's RX) from the RS232 side
- In addition it offers a low power mode whenever all RS232 side input signals are below +/- 3V.
Can anyone state definitively what voltage these run at (at the Psion end).
If 'Psion end' means the signals on the interface side (external connector), then it will put all outgoing signals at at least 5V (*1) with a maximum of 10V. It accepts input signals between 3V and 25V. If no input signal is above +3V or below -3V, meaning there is nothing connected or not active, it goes into a power saving mode, disabling all output signals. Quite handy for power constrained applications like a Psion.
Of course, 'definitively' values need to be checked at the device, as some early series of these chips were rather quirky.
*1 - 'Real' RS232 operates with negative 3..15V for space (logical zero) and positive 3..15V for mark (logical one). This is because it was originally defined for TTY like devices with less than perfect regulation operating over long lines creating considerable voltage drop. It's telecommunication, a complete different world than the usual fine regulated, narrow definition,