problem with direct interconnection with telit GM862

hi everybody

i’m working on making a direct connection between the rabbit RCM4200 and telit GM862 for commanding it by AT command but nothing happens no communication!!
i know that the two are on 3.2v this why i wanted to eliminate the max232 and working without but it doesn’t work

can someone give me a reason please !! :confused:

The reasons could be legion with the amount of data you have provided.
Have you tested that both devices have the correct supply voltage?
Have you tested that the rabbit does indeed output serial on the correct port?
Have you tested that both devices is talking at the same baud rate?
Have you tested that the rx pin is connected to the tx pin and vica versa?

For development I would first work through a max232 so you could monitor each device seperately with your PC serial port, then remove the max.

I use this device myself and it indeed works well with the Rabbit but you need to pay careful attention to the power and interface on this device.

OK. Number 1, the Telit GM862 is not 3.2 or 3.3V compatible, it is 2.8V so you can’t connect the 3.3V from the rabbit to it directly. I have it working by putting in a current limiting resistor to all input pins. Output pins are connected direct to the Rabbit and the logic level is sufficient for the Rabbit.

Number 2. What supply are you feeding the GM862 with? It needs to be a nominal 4.0V or it won’t work.

Number 3. Have you connected up the POWER ON-OFF control on the GM862 to a transistor and switching this from the Rabbit. This is how the device is power on. See the datasheet for the timing of this input. It can also be used to power the modem off.

Number 4. Watch the names of the pins. For instance, the TXD line is actually an input and is the TX data from the Rabbit. I got caught out with this one before. Always read the datasheet to determine the IO direction.

Number 5. What baud rate are you using? The default from the factory appears to be 115200.

Number 6. Is this a custom design for the GM862? If so, watch the power supply design as it has to be able to handle 2amp peaks during transmit. For data this will be very small but you need to make sure the supply is stable as otherwise the device simply shuts down if the supply drops below 3.8V or so.

Hope this helps?