In order to connect your Psion to your Mac, you’ll need a couple of bits of hardware: an RS232 adapter, and a suitable link cable.
RS232 adapters using an FTDI chipset are the most reliable on macOS and are recommended. This one from Amazon US seems to work well.
Important
The RS232 adapter you select must support the DSR/DTR hardware control signals to work with Reconnect and plptools. Prolific adapters can be made to work, but the default drivers that ship by default with macOS don’t handle these correctly so you’ll need to install the official drivers.
Alex Brown has done some pretty rigorous testing of different adapters which you can find on his Hackaday blog.
The link cable you need depends on the type of Psion you have. Unfortunately these can be a little expensive now, but Psionex still have stock at relatively competitive prices.
Note
These are commonly known as ‘Honda’ cables as Honda manufactured the original connectors.