RS232 cable and MAX232+AVR cable scheme problem

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#1
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Hello,

I am trying communicate with WAVECOM GSM modem over RS232 serial port

if I have serial cable connected between

PC=USB2COM <----> WAVECOM GSM MODEM I am able communicate with GSM modem

if I use reduction (Canon9 M-M)
3 wires used (signals GND/TX/RX)

PC-USB2COM <-------> MAX232--AVR
I am also able send commands AT to AVR
and AVR IC is responding

but if I use same 3wire cable between

PC-USB2COM --------- WaveCOM GSM modem

I am not able receive any response on 'AT'
command same is when ATmega connected to GSM modem

I spend 2 days with checking wiring
testing but I have no idea why I am not able
communicate over this 3 wire simple cable I dont have connected other signals on MAX232 level shifter so I am not able use DTR/CD/CTR or RTS/CTS

Currently I have PINs 7/8 on WaveCOM GSM modem side connected together as have some nullmodem schemes still is no response from GSM modem

Any idea why PC is able communicate
over full signals cable
and I am not able with 3 wire cable?

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In the PC software did you turn off "flow control"? If not a PC resolutely won't communicate without DTR.

http://en.wikipedia.org/wiki/Ser...

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there is primary problem I am not able
communicate between AVR board + levelshifter(MAX232) and WaveCom

I used example with PC GSM modem + same cable as example

but I will check if is disabled flow control but I newer used it I am using 9600speed only and there is not required.
I think


update
No Flow HW controll was set
its another issue I think I have feeling
I should to use also signal for WaveCOM GSM becouse this device is checking
I have tested AVRprogram with PC and I am not able continue now :( becouse GSM modem is not responding :(

Last Edited: Thu. Nov 24, 2011 - 03:55 PM
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Well another alternative - cross Tx and Rx in the 3 wire cable. In theory a modem is a DCE and the PC is a DTE so it should work with a "straight through" cable with Tx-Tx and Rx-Rx

(of course you can buzz out the multi-wire working cable and see how ipns 2 and 3 are connected)

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(check also previous post I updated it in same time when you post your post)

I tested this first
I spend 2 days playing with cables

I tested RX-RX - TX-TX(straight) or RX-TX RX-TX (cross)

I tested send blindly AT command sequence which change GSM modem LED/lighting/ enter PIN and log into GSM network=LED/flashing/

This also not working so modem dont received any command and I have not response also

when I unplug GSM modem canon9 cable and plug it to PC I see serial communication
cable is good only GSM need some special wiring :(

I forgot say something important
GSM modem is using high density 15pin canon
then short original cable with canon9pin then is my Male-Male 3 wire cable between AVR and GSM

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I will try morning this
http://coderstalk.blogspot.com/2...

I am not alone who has this problem

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This is in fact, a very old "problem". It is as old as RS-232. It is the issue of "who transmits and who receives on what pins".

One way to tell is to check voltages with a volt meter. An input (that is, Rx) will measure close to zero volts with respect to common. A output (that is, Tx) will measure +3 to +12V (typically) when nothing is being sent. RTS, CTS, DTR, & DSR outputs are just the opposite (-3 to -12V) when inactive.

Check Wikipedia for details:

http://en.wikipedia.org/wiki/RS-232

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Jimm its solved with this URL which I post
disabled

+IFC: 0,0

OK

and 3 wire cable seem to be solved.
clawson has good comment check Flow control
I have it disabled but modem has it enabled
now is disabled and saved to FLASH conf memory with AT&W

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Problem solved with following cable + disabling flow controll on Wavecom modem side probably helps to someone in future

Howto wire MCU and Wavecom Modem with 3 wire serial cable


canon 15pin to WaveCOM modem 
__________________________________
\  o    o    o    (B)    o       /
 \    o   (C)   o     o     (R) /
  \ o   o    o     o     o     /
   ____________________________
         (soldering side)

B Blue  cable  <---->  B1
C BlaCk cable  <---->  C5
R Red   cable  <---->  R3


Canon 9pin DB9 connector to MAX232/MCU
__________________________________
\  (C5)  o     (R3)   (B1)   o   /
 \    o     o       o     o     /
  \____________________________/
         (soldering side)