I'm setting up a pretty standard RS232 interface from a digital caliper to a microcontroller, via a MAX232 chip (or in my case, MAX3222, which is the 3.3V-supply variant with 2 transmitter pairs and 2 receiver pairs).
The only thing non-standard about this particular caliper though is that it uses an Opto-RS232-cable, which specifies connections as follows:
The RXD and TXD connections are obvious.
And DTR is for [Power +] so I connected it to T2-OUT on the MAX chip, thus I can write a HIGH on a MCU pin connected to T2-IN ==> thus T2-OUT will be at +5-15V.
Thus, my connections so far look like this:
Is there a simple solution for the one remaining connection: RTS?
It's for [Power -] and requires an OFF/LOW of voltage level -5 to -15V.
But unfortunately, there are no more RS232-receive-pins left (T1-OUT and T2-OUT are taken already). I could use a chip like MAX3246 with even more channels but that would add to BOM cost.