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Old 03-14-2011, 10:13 PM   #1
cucub
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Beginner at ttyS1


Hi folks,

I'm a quite "old" linux user but new on serial communications.

Following my current problem:
I have a embedded system from Via (vx855) equipped with two setial ports. The second port, ttyS1 is set to RS422 by jumper on the main board. This port is conected to a stepper motor. Now, by this connection I should be able to communicate with the firmware of the motor. But somehow it does not work. Using a USB-to-RS422/485 adapter and "talking" (putty, screen etc..) to ttyUSB0 works fine but on ttyS1 not.
I used the serial port sniffer jpnevulator to get an idea what is going on: when I type eg. "q" to the ttyS1, jpnevulator sniffs a "G" (ASCII 47).... looks like all keys have been mixed up.... Hove no idea where to search for this problem, any advices?

Example:
echo echo -e -n "q" > /dev/ttyUSB0 sniffer reads ASCII(HEX) 71 with is q
echo echo -e -n "q" > /dev/ttyS1 sniffer reads ASCII(HEX) 47 with is G, how come???

Last edited by cucub; 03-14-2011 at 10:37 PM. Reason: Example added
 
Old 03-15-2011, 03:33 PM   #2
Rod3775
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The most likely problem is an incompatibility in serial baud rate or stop bit (less likely) between your devices. You didn't say what baud rate you were trying to use, but a stepper is not going to want anything real fast. If you don't have a direct way to measure baud rate, you can send a long sequence of chars (like 1000) and stopwatch how long it takes (and that the # of characters received by your sniffer is correct!). Another possibility is reversed differential signals. RS422 is a balanced voltage system, and if your embedded system drives the lines in the opposite sense than your USB converter, you would get garbage. Check the polarity from each (with a voltmeter) when nothing is being transmitted, and make sure they are both the same.
 
Old 03-16-2011, 09:59 AM   #3
cucub
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Hi and many thanks for the answer

Baudrate is both 9600, stop bit both 1

Quote:
Originally Posted by Rod3775 View Post
Another possibility is reversed differential signals. RS422 is a balanced voltage system, and if your embedded system drives the lines in the opposite sense than your USB converter, you would get garbage.
That looks like a very good guess. Just got the RS422 pin configuration from the embedded board:
Embedded board RS422 D-Sub9:
Pin 1: TX-
Pin 2: TX+
Pin 3: Rx+
Pin 4: Rx-
Pin 5: GND

USB-to-RS422 converter RS422 D-Sub9::
Pin 1: TX+
Pin 2: TX-
Pin 3: Rx+
Pin 4: Rx-
Pin 5: GND

So TX seems to be reversed.
I gone make a new wire, test and report....
 
Old 03-20-2011, 01:43 AM   #4
cucub
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Tested and yes, TX is reversed, now all works

Many thanks
 
  


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