Changes

This interface was originally described in the book "[[Maschinenspracheprogramme und Hardware-Erweiterungen für Schneider CPC's]]" written by [[Paul Bauriedl]], released by IDEA Verlag in 1987. Around mid-nineties, Tim Riemann has it was turned into a single package and circulated the schematic as GIF in the internet.

The circuit is nearly indentical to the [[Schneider RS232 Interface]] from 1985, both circuits are using a [[Media:Mostek_3801_Data.pdf|Z80 STI]], both mapped to the same I/O port addresses. Both circuits seem to be are driven by the 4MHz signal on the CPC's expansion port, so even and connect CTS/RTS/DSR/DTR to the the timings should be same pins, so they are (almost) 100% compatible. Some differences:* The only difference is that the DIY Schneider circuit uses more logic chips for decoding of the I/O port addresses (the extra chips are just more over-complicated, not actually better)additionally implements CHRDET. Another difference, concerning the power supply, is that the * The Schneider circuit contains 2 diodes and 2 diodes capacitors for converting ~7V 8V AC to +8V/-7V 8V DC.* The Schneider circuit uses only 2 chips for port decoding (the overcomplicated DIY uses 5 chips for exactly the same purpose)

Image:Tim Riemann RS232.gif|Tim Riemann's Z80 STI Schematic (1996)

Note 2: The 74LS138 splits the address space to four regions (which is total nonsense), the 74LS20 is then merging them back to a single region (which is undoing that nonsense). There seems to be no real reason for doing that - except maybe to avoid legal copyright problems (claiming something like "no Although the 74LS138 is rather useless in this isn't a unlicensed remake of Schneider's schematiccase, our schematic is doing total nonsensethe book from Paul Bauriedl contains further schematics, it's has absolutely nothing to do with Schneider's hardware")in which the 74LS138 does have some purpose.