Re: Apple IIGS Serial Input

[dempson@actrix.gen.nz (David Empson)]

Smohn Jith <Smohn@smohn.com> wrote:

> I have a device that has 3 wire RS232 serial.  TxD, RXD, and Ground. 
> I'm trying to get it to work with my IIGS.  I succeeded in this 10 or
> more years ago and I forgot what I did.  I have the 3 wires hooked up
> correctly, but I vaguely remember having to ground the handshake pins or
> the General purpose input pin or something.  I've tried a bunch of 
> combinations without luck.  Any ideas?

The IIgs serial port uses RS-422 for its transmit and receive lines.
RS-422 has a balanced pair of signals: TxD+ and TxD-, and RxD+ and RxD-.

If you are hooking up an RS-232 device, you use the TxD- and RxD- pair
on the IIgs. You can ignore TxD+, but you must connect RxD+ to ground,
otherwise the receive signal doesn't work properly (it is comparing the
voltages on RxD+ and RxD-, so uses the ground on RxD- as a reference).

<http://apple2.info/wiki/index.php?title=Pinouts#Mini_DIN-8_Serial_Port_
.28GS.2C_IIc_Plus.2C_Macintosh.29>

-- 
David Empson
dempson@actrix.gen.nz