Hitting me with the serial port

Projects Networking, Protocols, and Devices
1

Hi guys,

I've wasted many days with a problem with the serial port.

I'm using Mega2560, MAX232 (DB9) and WAVECOM Fastrack GSM modem (DB15).

  • My MAX232 connects fine with PC-RS232. I open two TeraTerm windows (one connected to Arduino, another one to PC serial port) and no problems sending and receiving.

  • No problems connecting PC to modem with original cable (DB9 to DB15).

  • I have an old 56K modem. I tried the MAX232 and runs smoothly.

But ... I can't connect the MAX232 to the GSM modem.

  • To connect MAX232 to GSM-modem I used a three-wire cable TX/RX (2/2), RX/TX (3/6) and GND/GND (5/9). Where (X = DB9/Y = DB15).

  • I found a clue. I could connect from PC to modem using 4 wires. The fourth wire going from RTS (DB9 pin 7) to CTS (DB15 pin 11).

  • I discovered I can disable flow control on modem. The command is AT+IFC=0,0. It did not work.

  • I've also tried to tie CTS and RTS signals on modem. It did not work.

  • By the way, also tied DCD, DTR and DTS.

But still no response.

How I can have the "fourth wire" (CTS to RTS) from MAX232?

Any ideas? THX!

wires.jpg

2

Have you had a go with this?

http://www.lammertbies.nl/comm/info/RS-232_null_modem.html

Read through it and try the workaround shown there for the cable.

3

thxs bubulindo, I found a possible solution.

The connections are:
DB9 - DB15
3 ---- 6 (TX-RX)
2 ---- 2 (RX-TX)
5 ---- 9 (GND-GND)

If tie CTS to RTS does not work.

If I turn flow control to none (using AT+IFC=0,0) does not work.

But... If I connect pin 12 (RTS) to GND and pin 11 (CTS) to +5V... Surprisingly, the modem answers!

Will there be any problem with the circuit if applied +5V to CTS? Is it safe?