What I was doing wrong was having RTS/CTS ticked in the 'Handshaking' section of the Psion's Communication settings. Once I cleared that, and made sure _all_ the Handshaking options were unticked, it worked.

Hi there, Appreciate this is a five year old thread so apologies for being late to the party. I'm trying to get a Psion 5MX to talk to a Raspberry Pi 3 using a Null Modem Adaptor and a RS232 - TTL convertor. I was delighted when it appeared to work, see pic here: https://www.instagram.com/p/BcPEZsDj...

Thanks for sharing your experience.

My son and I just had the exact same problem. We solved it with the suggestion above of using a SDHC compliant USB card reader instead of the internal reader on my olde ThinkPad X60s.