All the RS232 interfaces I've seen that are based on the 6551 seem to use a MAX 232 as well, to do something the signals.
http://retro.hansotten.nl/6502-sbc/lee- ... acia-6551/- you don't need the MAX232 circuit if you only want a TTL interface (e.g:
connecting to a minitel)
- no +12/-12 needed, the MAX232 chip provides EIA compatible levels (9V)
- as the MAX232 chip only allows two inputs and two outputs, I had to choose
one input among DCD, DSR and CTS; and one output among DTR and RTS.
DCD seems to be the most important input if you connect a modem to the
interface (it tells you when you are connected), that's why I choose it.
Also, I have chosen DTR rather than RTS in order to tell the attached
equipment when the cpu is ready, and this signal is usually connected to both
DSR and DCD in a null-modem cable.
So possibly this Oric RS232 interface is only usable with the provided V23 modem?The MAX232 performs level translation between the TTL levels of the 6551 and the bipolar RS232 levels. Only the signals for a 5 wire interface are implemented with DCD and DSR being tied low on the 6551 to allow continuous operation.