Steve Walz of armory.com says "Use a TTL HI output for the RS232 *GROUND*!!
Then you get +/-5VDC levels to fake out the RS232. When TTL-GND goes HI,
then a TTL-XMT LO looks like -5VDC and a "mark". Even sign problems go away
in hardware! When TTL-GND goes LO, and TTL-XMT goes HI, it looks like +5VDC
and a "space"! You can derive the two TTL signals from one off an inverter
or inverter pair! Then magically, you have direct TTL to RS232 conversion!!

This has worked for me without any external chips by just copying the
inverted output to a second pin in software and connecting it to the TD and
the "original" pin to the Signal Ground. You might use a hex inverter (7404)
to avoid modifying the software. It complies with the RS232 spec (which
calls for at least +/-3v and not more than +/-12v)

Clear to Send (CTS): CTS is used along with RTS to provide handshaking
between the DTE and the DCE. After the DCE sees an asserted RTS, it turns
CTS ON when it is ready to begin communication.       Hard-wire to RTS on
connector to bypass this function

Data Set Ready (DSR): This signal is turned on by the DCE to indicate
that it is connected to the telecommunications line.       Hard-wire to DTR
on connector to bypass this function

Data Terminal Ready (DTR): DTR indicates the readiness of the DTE. This
signal is turned ON by the DTE when it is ready to transmit or receive data
from the DCE. DTR must be ON before the DCE can assert DSR.       Hard-wire
to DSR on connector to bypass this function

James Newton, webmaster http://get.to/techref
(hint: you can add your own private info to the techref)
mailto:jamesnewton@geocities.com
1-619-652-0593 phoneÊ



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@mitvma.mit.edu]On Behalf Of Dan Rosenfeld
Sent: Sunday, September 19, 1999 8:55 AM
To: PICLIST@mitvma.mit.edu
Subject: rs-232 w/o level convert?


Hi all,

>From looking at the archive, I can see that this is a somewhat FAQ.
Unfortunately, I'm having trouble assembling all that I've read into a
coherent picture applicable to my simple problem....

I'd like be able to send characters (debugging printfs) from the 16LF84 in
my circuit to my PC serial port W/O USING A LEVEL CONVERTER CHIP. I only
need this to work on my PC for debugging purposes (it's not a production
circuit) and at low baud rates and short cable distances. I'm also wondering
whether I can get away with running this all at Vdd = 3.0V (thus the LF84)
or will I need run at some higher voltage while debugging.

I'm pretty clear on the PC side (terminal emulator app or basic program),
but I'm not certain of the exact electrical connections on the pic side.
I've seen mention of using anywhere from 1k to 1meg in series w/o tx/rx
lines and along with some mention of pulldowns on these lines.

Also, do I need to do anything special with any of the other rs-232 lines
such as RTS/DTS, etc. or can I just let them float? (Assuming the pc
software is configured correctly.)

So, what's the *exact* wiring/circuit that I should use here ?

(BTW, inside the PIC I'm planning on using the CCS compiler's serial IO
routines.)

Thanks in advance,

Dan