> I've got a design that someone else gave me, for a PIC16C58B.  It has
> the rs232 serial lines from the db9 connector coming in PIC pins 1 & 2. 
> Transmit from the DB9 goes thru a 33k ohm resistor to PIC pin 1, and DB9 
> receive is tied directly to PIC pin 2.  DB9 pin 5 is tied to ground.  Comm 
> is 19200 baud, no handshaking (3 wires only).  I'd like to add a max232
> chip to the design.

Good.  Actually adhering to relevant standards (EIA/TIA-232 signaling
levels) is an excellant idea.

> I've got a max232 chip wired up to it, but can't get it to work.  The
> guy that sells me the (pre-programmed) PIC's says the PIC serial pins
> "are of oppposite logic levels" so the max232 won't work with it.

Essentially, you can think of the EIA-232 driver chip as providing
one signal inversion per pin.  Given the non-compliant way the PIC
was connected, the original design must do the inversion in software.

To continue using the PIC software unmodified, you need to insert a
logic level inverter on each of the TxD and RxD lines between the PIC
pin and the MAX232 pin.  74HCT04 or 74LS04 hex inverter would work.

> I've got the chip wired up similar to what's shown here:
> 
> http://www.boondog.com/tutorials/pic16F84/rs232x.gif
> 
> except I have C9 tied to gnd, not VCC.  What am I doing wrong, or will
> this even work without modding the PIC itself?

I believe you need to tie C9 to Vcc as shown.  It's part of
the voltage doubler/inverter system.

						Lee Jones

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist