GSM modem interfacing with microcontroller

[robert.guttormson]

I have been through another thread that deals with the issue of interfacing GSM modem with a microcontroller. In my case however, the eval board of the microcontroller comes with ST3232 chip. I am able to open hyperterminal and send and receive data using the UART. From the schematic i understand that only the RX/TX/Gnd pins are connected to the DB9 connector. The microcontroller also provides only the RX and TX signals. I am also able to successfully send receive sms by connecting the modem to pc via hyperterminal.

Can anyone let me know why my microntroller is unable to talk to the GSM modem. It is sending all the commands but doesnt receive any response from the modem.

TIA,
Bob

 

[Rusttree]

When you interface the uC to the modem, are you using the ST3232 chip?

Also, when you connected the modem directly to the PC, what did your circuit look like?

 

[robert.guttormson]

uc is connected to the ST3232 and the pins are connected to the DB9 female connector. I connected the PC to this DB9 female connector with straight UART connector. I tried connecting the same uc+st3232+DB9 to modem with a null modem adapter but didnt get response to the AT commands sent from the uc

 

[Rusttree]

Does the modem require RS-232 logic levels?

 

[robert.guttormson]

I am not sure of that, it is from a chinese manufacturer and has no documentation. However when i connected it to the pc via the serial port and hyperterminal i was able to send AT commands

 

[Rusttree]

So you're not using any voltage level translators between the PC and the modem? That would indicate the modem is RS-232. A google search of "GSM modem RS-232" came up with a lot of hits, so it's probably a safe bet that that's what you've got.

Most likely, then, the modem requires some specific serial protocol that your uC is not providing. HyperTerminal is probably very forgiving and will accept serial transmissions that the modem won't. Since you have no documentation, this will take some investigation to figure out. Do you have a digital scope? I'd try sending an AT command from the PC to the modem and scope the line. Then send the same AT command from your uC and see what's different on the screen.

 

[Ian Rogers]

This question has been here before..
GSM modems use hardware handshaking.... on the 9 pin connector on your micro board you need to connect together RTS and CTS pins 8 and 7, you could also connect DTR and DSR pins 6 and 4.

Then the modem will respond

 

[robert.guttormson]

If GSM modem expects full hardware handshaking shouldnt i be connecting the pins(DTR<->DSR/CTS<->RTS) on the DB9 connector on the GSM modem ?

 

[Ian Rogers]

It doesn't matter where you connect them... they crossover!! 8 to 7 is the same as 7 to 8 as long as rts is connected to cts and rts from the modem.

 

[Nigel Goodwin]

You can also send a simple command to turn off hardware handshaking - in my experience you don't need hardware handshaking in place in order to do that.

For my GSM chip the string required is:

"AT+IFC=0,0", CR, LF, 0        ; disable handshaking