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The Psion 3c and series 5 PDA computers have a 15-pin serial port at the back, which can connect to a PC serial interface via a special cable, or to a battery-powered modem.

I have read that the serial port was powered by a Maxim MAX3212 chip, which runs at around 3 V. However, it isn't clear whether TX and RX run at RS-232 12 V, or at 5 V or even at 3.3 V.

Can anyone state definitively what voltage these run at (at the Psion end)?

Edit: it appears from further research that there is a difference between the Psion 3/3a and the 3c/5/5mx. The former required a 'soap on a rope' cable which changed the voltages to RS232 standard, the latter seems to directly output the correct voltages. This ambiguity led to my original query. I have updated the question to limit it to the 3C onwards.

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2 Answers 2

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(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,

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  • Thanks. I meant the Psion end of the PC cable, in cases where there is a voltage level change circuit.
    – Mark Williams
    Commented Apr 18, 2022 at 22:42
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    yep MAX chips includes capacity based voltage multipliers that create the +/-10V from 5V or 3.3V ... beware early batches of the unipolar versions where unreliable and burn a lot (after they convert it from TTL technology ... the clones where as bad too)... old TTL and new unipolar chips are reliable ... Also cheap (USB) RS232 converters usually do not use MAX and have non standard voltage ranges like -3/+7 V and sometimes will not work reliably with RS232 compliant USARTS
    – Spektre
    Commented Apr 19, 2022 at 6:20
  • @Spektre true - I added the remark - but I believe the ones used with the Psion are already mature types.
    – Raffzahn
    Commented Apr 19, 2022 at 9:09
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    Acceptable receive voltage must work down to +/- 3V, but acceptable transmit voltage is larger, +/- 5V minimum. And negative voltage is logic one and positive voltage is logic zero.
    – Justme
    Commented Apr 19, 2022 at 10:00
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    @Spektre Not all RS-232 transceivers use a capacitive charge pump. This specific MAX3212 for example uses an inductor based switch mode power supply to boost voltage to about 6.5V.
    – Justme
    Commented Apr 19, 2022 at 10:04
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Yes it does work at RS-232 voltages. At least on a Psion Series 5 device, the MAX3212 RS-232 transceiver is inside the Psion, so the Psion will directly use RS-232 voltage levels.

The actual voltage levels are irrelevant because they are compliant with RS-232 specification, but according to MAX3212 datasheet, it can output at least +/- 5.0 volts and typically +/- 5.5 volts when all outputs are driving receivers with a worst case load of 3 kilo-ohms.

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  • That was exactly what I needed to know.
    – Mark Williams
    Commented Apr 19, 2022 at 16:53

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