The max233 has an internal charge pump circuitry to get the +10/-10 volts for
the RS232 connection. This may be part of the explanation ?

Quoting Bob Barr <bbarr@CALIFORNIA.COM>:

> I'm using a Maxim 233 chip for the RS-232 interface on a small 16C745
> project board that I've built. Somehow, with my supply switched off
> and the RS-232 port connected, my Vdd line is being held up to
> somewhere between 2.3 and 4 volts depending on current load. (The
> short circuit current being delivered measures at about 26 mA.)
>
> I originally thought that the Vdd was coming straight back out of the
> 233 so I first tried putting a 1N4148 diode in series with its power
> pin. No help. (It does show a 0.6 or so forward voltage drop whether
> my supply is on or not.
>
> Another possibility that I considered is that the PIC's protection
> diodes could be passing the voltage through from an IO pin. Nope,
> removed the PIC, no change.
>
> I've ohmed out the circuit and didn't find any shorts. In case an
> unused pin was somehow at fault, I bent out all of the unused pins of
> the 233 at its socket.
>
> With power off and enough load to get down to 3.60 volts Vdd, the
> 233's pins are at the following levels:
>
> pins 1 and 2 (T2in and T1in) +2.92 (I've got these tied together)
>
> pin 3 (R1out) +2.95
>
> pin 4 (R1in) -11
>
> pin 5 (T1out) -4.63
>
> pins 6 and 9 are ground
>
> pin 7 (Vcc) +3.60 before the diode, +2.96 after
>
> pin 8 (C1+) +4.17 (no connect, lifted)
>
> pins 10 and 16 (C2-) -2.38
>
> pins 11 and 15 (C2+) +2.42
>
> pins 12 and 17 (V-) -4.63
>
> pin 13 (C1-) +1.59 (no connect, lifted)
>
> pin 14 (V+) +5.23 (no connect, lifted)
>
> pin 18 (T2out) -4.36
>
> pin 19 (R2in) 0 (no connect, lifted)
>
> pin 20 (R2out) +2.95 (no connect, lifted)
>
>
> It's rather unnerving to have a bunch of LEDs stay dimly lit after I
> shut the supply off. I also really have to wonder if I'll be able to
> get reliable resetting of the PIC with the Vdd level kept so high.
>
> Am I missing something real obvious here? Any suggestions as to what I
> might try to eliminate this 'phantom' Vdd supply?
>
> Thanks, Bob
>
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