Re: Trying to send RS232 with PIC sends incorrect data in TeraTerm


<jbitz34@xxxxxxxxx> wrote in message
news:1176324076.120588.120280@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 11, 1:49 pm, "Roger Hamlett"
<rogerspamigno...@xxxxxxxxxxxxxxxxxxx> wrote:
<jbit...@xxxxxxxxx> wrote in message

news:1176252759.067057.96530@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> So we
shortened the message to just output "A".

The display on the Tera Term is now "_" (underscore)

The binary for A is 1000001
The binary for _ is 1011111.

Just an update.

Ok.
Now multiple things. If you have download rights for the compiler,
3.249,
is available to download. Install this into another directory, and see
if
the behaviour is the same. If so, you have at least ruled this out.
Now, remember that for asynchronous serial, the data is sent LSBit
first.
Also, the 'start', requires a '0' (low) at the input, before the data
byte
itelf is sampled. Then there is a high 'stop bit' at the end. So, 'A'
is:

1010000010x1, with '0' being 0v, and '1' being 5v (the 'x' is the
parity
bit, which would depend on whether this is enabled, and how it is set)

The _, is:

1011111010x1

Now the chip will start reading, when it sees a 0v for the first time.
Notice the five successive '1's in the second stream, and the five
successive '0's in the first.
If you invert the bit pattern for the 'A', you get:

0101111101/x0

Notice how if you shift this left one bit, it closely resembles what
you
are receiving.
It looks suspiciously like you might be lacking a signal inversion in
the
connection to the PIC (RS232 uses -3 to -25v for a '1', and +3 to +25v
for
a '0').
Are you sure the converter you have is USB to RS232?. You speak about
it
as being expensive, but these are incredibly cheap (well under 10$). I
have a nasty suspicion, that your converter, might be something like
USB
to RS485, rather than RS232!. This would explain it's price. While I
agree, that the USB-RS232 converters can sometimes be troublesome, most
now work OK.
I would try the simple test of connecting it to another PC with RS232
(can
you borrow one from a friend?),using just the minimum 3 wire connection
(TX to RX, RX to TX, and GND to GND). At least this will allow you to
'rule out' the unit if it works.
I really would be looking at the snalling polarity, and the connections
round the MAX232.

Best Wishes

Well I'm a poor college student, so 20 dollars is expensive to me ;-)

The data*** for the part indicates USB to rs232 functionality; and
the chip that resides on the breakout board is the cp2102 which lists
as its applications rs232 to USB on its data***.
Hint. Common mistake.
Same one in CCS.
They have their setup line called '#use RS232', but in fact it sets the
chip up to produce TTL asynch serial, which _requires_ the MAX232 (or
similar), to 'generate' RS232 signalling. In fact the same software setup
is used with different hardware, to provide RS485, or a number of other
similar standards.
The CP2102, is exactly the same. It _does not_ provide USB to RS232
functionality, it provides USB to asynch TTL serial, and also requires a
MAX232 or similar transceiver to generate 'RS232'. If you look at the data
*** for the chip, it shows an 'external RS232 transceiver' _after_ the
chip, to generate RS232 signalling levels, and signal polarity.
Now you are talking about a 'breakout board', which suggests you are
accessing the CP2102 directly. If so, it's connections, want to go
_directly_ to the PIC, without the MAX232 transceiver. This would explain
the signals being inverted, since each MAX232 inverts the signals, as well
as shifting the voltage levels used, and you have one missing. You need to
either run with none (TTL signalling), or with _two_ - RS232 signalling
between these. Check the voltages being produced from the 'breakout box',
with the scope. I suspect you are going to find these are basically 5v
levels. If so, get rid of the MAX232. I just hope you have not damaged the
chip, since the MAX232, delivers +/-12v signalling, and if I am right, you
have been feeding this into a chip rated for 5v signalling...

Here is the sparkfun product page:
http://www.sparkfun.com/commerce/product_info.php?products_id=198
Note that this _correctly_ says this is a _USB to serial_ adapter,_not_ a
USB to RS232 adapter. Get rid of the MAX232 on the PIC.

I am not sure what snalling polarity is. Our connections to the
Max232 have been checked by many people, and we have done so according
to the numerous schematics we have found. Also, we know that the Max
works at least for data sent out from the PC and shorted back 'round
through the pins.
The problem is that you don't need the MAX232, since you are not using
RS232 signalling anywhere...

Best Wishes


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