About RS232-to-PIC communications,

I'm a total novice when it comes to analog circuitry, so this probably isn't
the most elegant of solutions.

I needed a simple, robust, low-power RS232 connection for my data logger. I
minimised the components on the logger by using an "active" plug. It works
like this:

The logger itself has only two connections: GROUND and DATA. DATA goes
straight to a PORT-B pin on the PIC through a resistor.

The plug contains an op-amp (eg 741) powered from the DTR and RTS pins via
diodes (for protection). One input of the 741 sees a voltage of around 0.6
volts (actually, one diode-drop above ground). The other input is connected
to DATA. Receive (from the host) is connected to DATA via a resistor. Transmit
(to the host) is connected to the ouput of the OP-AMP. You can fit the whole
arrangement (OP-AMP + a few diodes) in a 9-pin backshell.

Advantages are:
(1) No active components on the PIC board. All power is drawn from the host.
(2) The output of the OP-AMP swings to the supply voltages, so you get whatever
output voltage range the host uses (+-5v, +-12v, etc.) minus about 0.6v.
(3) Only 2 wires connect to the PIC. For a logger like mine this is a definite
plus. You can put the PIC inside a canister and communicate through the casing
(arranged like a lithium cell) - no sockets required.

Disadvantages are:
(1) The link is half-duplex. Every character sent by the host is echoed by
the interface.
(2) You need to be able to control the handshaking lines to some extent. On a
PC this is no problem.
(3) Its a little wierd, but sometimes wierd is the only way to go.

Hope this helps someone out there...

Stewart Smith
ssmith@csuvax1.murdoch.edu.au