Cellular Radio Tx/Rx ambiguity in schematics

Our cellular radio module works great, but while in the process of trying to design a board around it, I noticed that the schematics apparently have TXD wired to the Gadgeteer Tx pin, and RXD wired to the Gadgeteer Rx pin. According to the Gadgeteer docs, Gadgeteer Tx is data coming from the mainboard, to the module. From reading the SIM900 documentation, TXD is data sent from the SIM900 module, and RXD is data sent to the module.

Shouldn’t TXD be wired to Gadgeteer Rx, and RXD wired to Gadgeteer Tx? Which documentation is wrong? The Cellular Radio schematics? Or the SIM900 manual?

I did not check the schematics or the module, but I can tell you from previous experience this is common thing with GSM modems. They label TX as meaning it should go to Tx of you PC, instead of meaning that it is Tx into the modem. So it looks like Tx is connected to Tx and Rx to Rx. I’ve seen the same thing on several different GSM modems from different manufacturers. So its probably right the way it is on the schematics.

I’ve screwed myself up many times with this “problem”. You have to look at it as a host/client bridge situation. The CPU(Gadgeteer) is the host and the modem(GSM module, bluetooth module, etc) is the client bridge. The client bridge is expecting to be talked to by the host then pass it on to what it’s talking to for the CPU. The modem is simply a link completing a path of communication to the end point. It’s always best to look at the module documentation though as even engineers forget this and create a module that goes against this thinking.

There’s a distinction between DTE (Data Terminal Equipment) and DCE (Data Communication Equipment) devices.

PCs are configured as DTEs, where TX is an output and RX is an input.

Modems are typically configured as DCEs, where TX is an input and RX is an output.

As @ KiwiSaner mentioned, this indicates which ports [em]on the PC[/em] that they should be connected to, rather than a description of the modem’s ports. This convention was adopted because DTE devices are considered masters, whereas DCE devices are slaves, so typically the notation is adapted to be in relation to the master. Unfortunately, not all manufacturers follow this convention, so it’s advisable to check input/output direction for any serial device.