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.=20

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.=20

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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