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:

enter image description here

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:

My connections


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.


1 Answer 1


It looks like the opto-cable is resuing DTR and RTS as it's own power input pins.

Your MAX3222 uses charge pumps to generate the high positive and negative voltages for signalling over RS-232.

enter image description here

To me, if your cable doesn't draw a lot of current, the easiest thing to do would be to connect the positive supply (DSR) to V+ (pin 3) and the negative supply (RTS) to pin 7 of the MAX3222. Voila, power.

If your cable needs more juice, you'll need to provide it by some other means (a separate power supply or charge pump).

  • \$\begingroup\$ Neat idea, thank you. I'm going to try this and get back to you! By the way, what parameter on the datasheet would specify the amount of current that can be drawn from the V+ and V- pins? Here is the link to the datasheet. \$\endgroup\$
    – boardbite
    Mar 28, 2013 at 16:29
  • 1
    \$\begingroup\$ It doesn't appear to be explicitly specified. The charge pump will cut in if the capacitor voltage drops below 5.5V and cut out above 5.5V. You may need to play with the capacitor sizes to make this work. \$\endgroup\$ Mar 28, 2013 at 20:22
  • \$\begingroup\$ This method worked successfully (in my case, capacitor adjustment from the standard 0.1 uF was not necessary). \$\endgroup\$
    – boardbite
    Apr 2, 2013 at 2:33
  • \$\begingroup\$ @Inga That's great news. \$\endgroup\$ Apr 2, 2013 at 12:03

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