Telit GM862 problem [URGENT]

Wireless & IoT
1

I’m trying to test gm862 gsm modem by connecting it to PC, but it simply isn’t working/responding.

I’ve connected TxD and RxD to MAX232N and I’ve used two zener diodes due to gm862’s voltage levels. And after I’ve hooked up the hardware I’ve sent certain AT commands, but I don’t ANY response.

So, in order to determine what is the problem I’ve disconnected everything form gm862 except for power supply and saw that when I turn on the modem, TxD pin (pin37) goes ‘high’ for a second or less and then it goes back to ‘low’, although it should always be ‘high’ when the TxD line is idle.

What the problem could be? Please help ASAP.

Thanks,

johnny

2

How on earth did this topic become a sticky in Wireless/RF and GPS??

In future don’t cross post, it never gets you more useful replies.

3

gussy:
How on earth did this topic become a sticky in Wireless/RF and GPS??

In future don’t cross post, it never gets you more useful replies.

He selected “sticky” when he posted this topic…

Don’t know if the OP can “un-sticky it” but should if he can…

And, don’t double post in the future…

Back to the topic in hand, do you have your Tx/Rx lines swapped?

What are you using to power your project?

4

As power supply I’m using battery from another mobile phone (3.8V, 600mAh)

But like I said before, I connected power supply to the modem, and after powering it on, the TxD line should be ‘high’ but it lasts only for a second.

What can be the problem?

5

Have you checked the RxD/TxD connections, as has already been suggested? The GM862 transmits on RxD and receives on TxD, like most modems.

Leon

6

Yes I have.

But after I disconnect all connections, after powering on the modem, should one of UART lines be ‘high’?

7

RS-232 idle state should be ‘mark’ - negative. That would be high on the GM862 output, as the driver inverts the signal.

Have you got it on your own PCB, or are you using the SFE breakout board?

If you can’t communicate with it there must be something wrong. You could try contacting Telit support, I’ve found them quite helpful.

Leon

8

What are you doing with the DTR and RTS lines going into the module? You can try connecting the module’s DSR line to the DTR and RTS lines to let the module know that your PC is ready to communicate with the module. Also, it is a bit gutsy to connect a 3 volt device to your PC’s serial port (even with zeners). With a conforming RS-232 interface, the voltage levels go both sides of zero (+/-). I don’t think I’d risk my module in that way. Best to use a real level converter.

9

I’m using the similar GE-863 modules with just TxD and RxD. The other signals shouldn’t need to be connected. Handshaking can be disabled on the PC.

He has interfaced it with a MAX232. I’d use a MAX3232 at 3 V, which won’t need the zeners and should be OK with the GE862 logic levels.

Leon

10

leon_heller:
I’m using the similar GE-863 modules with just TxD and RxD. The other signals shouldn’t need to be connected. Handshaking can be disabled on the PC.

That’s good information to know. Not having either module, I put out the idea of checking the DSR/RTS lines as a suggestion.

leon_heller:
He has interfaced it with a MAX232. I’d use a MAX3232 at 3 V, which won’t need the zeners and should be OK with the GE862 logic levels.

My mistake. I completely missed the part about the MAX232N.

11

I’ve assumed he’s using the SFE breakout board. I’ve got one with a GE862 on it, but haven’t tried it.

Leon

12

I’m using mikroE dev. board.

The thing is simple: I connect only power supply, then I power on the modem and TxD pin is high only for a second, instead of being constantly high.

**Is there anything I can do?**

One more thing: which baud rate I should use when I communicate with the modem for the first time?

13

I use 38, 400 baud with the GE863. They have autobaud, so it shouldn’t matter.

Leon

14

Can you try to disconnect all hardware form your modem and see if TxD will be high after you power on the modem, it would help very much!