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Tips and questions relating to the GPS modules from SFE
By Alaris
Hi everyone,

I have a GSM862-GPS and breakout board and I am hoping to get it communicating over UART with a Freescale HCS12X microprocessor (this one specifically)

The GSM powers on fine, I have an LED hooked up to the PWR_CTL pin and when I power on it stays lit.

My level translation circuit is working fine. The Freescale uses 0-5v and the GSM uses 0-2.8v. I use a voltage divider (470/560) for Freescale TX and it should accept the 0-2.8v from the GSM on the RX. I have tested this by sending the command AT<carriage return> through the Freescale then through the divider and then looping it back to my Freescale's RX. The Freescale correctly reads the AT<carriage return>.

I have an oscilliscope hooked up and triggering from the GSMs RX line. (The manual mentions that RX and TX are named from a software standpoint, so RX on the GSM is its output). I turn the GSM module on, then when I press a button on my Freescale it sends AT<carriage return>, however I do not get a response from the GSM (confirmed on the oscilliscope). I would be expecting to recieve OK, but I get nothing.

I haven't seen many topics on getting stuck here, has anyone gotten past this point (GSM<>microprocessor over UART)? Does anyone have any ideas/suggestions as to the problem I'm having? I would love to get basic communication between the two devices working. I currently believe it could have to do with some of my UART settings on the Freescale. I'm currently using 38400 baud.

Thanks for your time,
By Alaris
Hello again,

I've noticed if I put a SIM card into the device it will send out data over its RX line. It will not, however, respond to any of my commands I send it through TX. Is it running a python script or something instead of being in "microprocessor AT command" mode? If so, what is the solution to that?

Thank you,
By DannyDJ
Hello! I have the similar problem with my GM862! I'm trying communicating with GM862 over PC so i've used MAX232 chip for this! So i don't if my TELIT is damaged or what, because the status led immediately starts flashing and PWRMON pin is also immediately ON even if i dont press ON/OFF button. But RESET-ing the RESET PIN works and the STATUS LED flashes. I wrote program in VB.NET for sending AT COMMANDS to GM862. Couple of times i got some trash from it after sending AT command but nothing like "OK" response.

Colud you please tell me hot is your start sequence after applying power to GM862? Do you have to press the ON/OFF button to start the TELIT?
User avatar
By leon_heller
I'm using the 863, but it's similar. You have to use the on/off button to start the unit. Here's the (PIC) code I use to get the initial response:
Code: Select all
		;initial check (100ms timeout)
		call	ClearBuffer
		call	OUTMSG1		;send ATE (no command echo)
		call	Timeout100ms
		call	ReceiveSerial	;get char in W
		movwf	INDF		;save in buffer
		movwf	Temp
		incf	FSR,F		;bump pointer
		btfss	Flags,0		;timout flag set?
		goto	main1		;no, try to get more chars
		call	dly		;yes, continue
By DannyDJ
Hello! Thank you for your response.I don't know what could be wrong. After applying power to GM862 it selfstarts without pressing the ON/OFF button, STATUS LED flashes(sometimes flash for a long period of time but sometimes just flashes couple of times and then stays on like it would during a CALL). When i try to shutdown wiht ON/OFF button it doesn't PWRCTL is on all the time. But if i manually RESET it it does reserts.
I'm clueless right now.

Maybe it has something to do with ADC pin which in HW user guide says that has to be grounded, but somewhere i found that BASIC 50 pin board has failure beacause this pin is grounded(I'm using that board but i made it my self). Maybe it has to do with RTS pin which is mentioned that has to be grounded if application uses only 3lines to communicate with TELIT.

Could you please describe your configuration for TELIT?
User avatar
By leon_heller
I don't think it should self-start. Does it switch off if you hold the switch down for two seconds? If it doesn't, there must be something wrong with the module.

GSM TxD on the module is an input, and RxD is an output. You must use a max. of 3.3V on the serial pins. All the other modem pins can be left unconnected.

The 862 might be a bit different.

By DannyDJ
No it doesn't swithc off and PWRCTL pin is also on which represents if the module is powered and it should be also low if you switch off TELIT. Only RESET works. On MAX232 i'm using two zener 3.3V diodes for lower levels from 0v/5v to 0v/2.8V level for TELIT.
By DannyDJ
Me again! Today i tried my TELIT again and i HAVE managed to switch on with ON/OFF button two times than it started to behave like before. Now i think that my board isn't so good.Maybe it has some bad connections. SO i will now make a new one only with pins that have to be connected for TELIT to work and to comunncating.
User avatar
By leon_heller
It does sound as if you have a hardware problem.

By zlockard
While I'm sure the OP appreciates everybody hijacking his thread and talking about only slightly related problems, there is still his post that nobody has responded to.

I am having the same issues as the OP, and would appreciate some input into the situation, as it is holding up my work a great deal.

Is the GSM supposed to respond with OK at this point, or is there possibly some other configuration that I (and the OP) have missed?
By velocet
Do you still have the hardware flow control set on the module? It is by default. This may give you problems.

You can check your level conversion by hooking it up to the GPS raw data output just to check that its working.
By Alaris
How do you disable hardware flow control on the GSM? Does it involve flashing the device? I searched these forums and couldn't find anything, I'm going to continue searching through the GSM documents and I'll update if I find anything.

I didn't realize flow control was on by default, this could very well be the problem b/c I have none of those lines hooked up.


edit: just found this in the manual, trying it out (whoops):
RTS should be connected to the GND (on the module side) if flow control is not used
By Alaris
Well after grounding that RTS line and testing out a few different baud rates, I still cannot get the GSM to respond to an AT<carraige return>. I am thinking of purchasing the USB eval model from sparkfun, confirming it works using hyperterminal, then begin again trying to get it communicating with a microprocessor.

Thanks for everyone's help
By velocet
AT\Q[<n>] Set command controls the RS232 flow control behaviour.
0 - no flow control
1 - software bi-directional with filtering (XON/XOFF)
2 - hardware mono-directional flow control (only CTS active)
3 - hardware bi-directional flow control (both RTS/CTS active) (factory
Note: if parameter is omitted, the command has the same behaviour as
Note: \Q’s settings are functionally a subset of &K’s ones.

Download the latest command set from

If you dont use flow control make sure you impliment a check some for data transfer AND NEVER TRY A FIRMWARE UPGRADE WITHOUT FLOW CONTROL
By Alaris
Well I was going to do 3 pin serial first before setting up all the hardware flow control as I thought it would be easier, but it sounds like I'll have to start with full flow control anyway. How would one send that command to turn flow control off without using flow control when sending that very command?

I'll test some more things out and report if I get anything working. Thanks again!
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