Getting the ZX Spectrum online with a Maplin RS232/Modem interface from 1983

Categories: Retro computers

Over the Christmas Holiday period managed to snag an original, part assembled ZX Spectrum RS232/Modem Interface from ebay, originally sold by Maplin back in the early 80’s. I went for this specifically since the ROM was included (because the Spectrum doesn’t use ASCII it’s necessary to boot from an external ROM which contains a translator and some pretty basic terminal emulator software.

The good news is that the original article on the design, assembly and a pretty thourough write-up with schematics is still available on the americanradiohistory archive (see Electronics, The Maplin Magazine Sept 1983). For completeness I’ve also scanned and attached the errata document (below) which faded was included with the PCB and parts sent to me (not necessary but still good to have). Well after a considerable amount of rework on what was initially a very poorly soldered PCB its back together. I sourced the missing ‘glue logic’ IC’s locally and gave it a shot – it works well…

To test this I’ve hooked it up to a WiFi Modem (running ZiModem CoCo firmware) and connected to the A80sAppleIIBBS. Note that there’s no flow control here, so found I needed to use the command

ATS44=30

to slow transmission down so that the speccy could keep up with incoming data.

Other resources

Future thoughts

Next steps are to get the speccy online with my Prism VTX5000 modem (which again contains and boots from a custom ROM).