Apple /// Serial card weirdness

The link below talks a little about the card.  At the bottom is mentions the button.  Apparently the button makes it act like a modem eliminator cable is being used?  I suspect that changes the pinout at the port to turn a regular cable into a null modem cable perhaps?  I'm not sure though.

https://www.apple3.org/Documents/Technotes/TA27977.html

nick3092 wrote:

Apparently the button makes it act like a modem eliminator cable is being used?

Indeed the layout shows all signals from the RS232 connector are routed to the switch - so the switch is obviously there to switch the pin mapping of all signals. Neat. So you only needed one type of cable - and could use it to connect to a modem (a DCE device) - or to another computer (a DTE device). Instead of using separate crossed (Null-modem) cables or non-crossed cables, you simply switched the card... I've never seen an RS232 card with such a feature...

The electronic looks the same like the onboard Rs232 (same controller). So, I guess it's likely software compatible - except for a different (base) address.

Verault wrote:

Do we have a list of the base addresses (board and card) I can reach out to the ADTPro forum and see if they can tweak the SOS adt client to add support for the card since apple /// support is limited.

The serial interface is described in the Apple III Device Driver Writer's Guide. The controller of the onboard serial interface is mapped to C0F0 (C0F0-C0F3). I/O slot #1 uses base address C080, slot #2 C090, slot #3 C0A0 etc. I guess the controller of the serial card is simply mapped to the slot's I/O space.

If you know what you're doing, you could probably patch the ADTpro binary yourself, replacing the C0F0 address with C090 (for slot #2, as an example).