I make project by use PIC communicate to GSM Modem by use MAX232
When I test RS232 port with PC, the board is work OK.
But when I connect board to GSM Modem board was broken.
I check voltage between ground (on pin5 of GSM modem and Pin5 on board's 232 ports) is about 125 Vac. and 0 Vdc. and I see Vac in every pin on GSM modem.
Why the modem has AC in the pin? and How to correct it?
If I you isolate circuit it will OK?
Could you recommend circuit for us. (I not use CTS and RTS with Modem)
Hi pecpys!
Your problem is definitely a grounding error.
> When I test RS232 port with PC, the board is work OK.
Beacuse in this case the grounding is correct throught your serial cable.
> I check voltage between ground (on pin5 of GSM modem and Pin5 on
> board's 232 ports) is about 125 Vac. and 0 Vdc.
> and I see Vac in every pin on GSM modem.
> Why the modem has AC in the pin? and How to correct it?
This points to the problem: you are worked here without a common ground line.
The correction: make it!
Whereby? I don't know your circuits/cables, you can solve the problem :-)
Probably (but insecure :-) you must interconnect the grounds !!!
> If I you isolate circuit it will OK?
Yes, the isolating is an other solution.
However, to make this, the schematics are a must.
You can see a lot of examples on the net.
Hope this helps!
zuisti
