Using LOOPBACK Testing with E-TEC and EV-Diagnostics

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jimh
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Using LOOPBACK Testing with E-TEC and EV-Diagnostics

Postby jimh » Sat Apr 28, 2018 11:06 am

A general method for testing serial data communications is called a LOOPBACK TEST. There is advice in the HELP section of the EV-Diagnostics software which describes how to perform a LOOPBACK TEST.

I presume most readers here are boaters and not serial data communication engineers and might find this short tutorial on LOOPBACK TEST procedures to be useful:

http://www.ni.com/tutorial/3450/en/

The integrity of the data circuit in a connecting cable between a terminal and the point of connection to the E-TEC EMM can be tested by connecting the RX and TX pins at the connector for the E-TEC, but that is really not a valid LOOPBACK TEST because it would fail to detect a wiring error in the cable itself: if the pins were wired backwards, this simple LOOPBACK TEST would not detect that error.

A LOOPBACK test of a terminal at the serial connection on the device can also be performed. Usually a mating DB9 connector is used and a connection between the TX-DATA and RX-DATA poles is made. This will verify the terminal and application software are talking to the serial data port.

In the data communication system used in connecting a computer terminal to an E-TEC EMM, both devices are wired as DATA TERMINAL DEVICES. This means that the cable connecting them must be wired as NULL MODEL CABLE. This simply means that the RX and TX circuits are crossed in the cable, connecting the RX of one device to the TX of the other, and vice versa.

If a cable is miswired, the will be no communication possible. This is the most fundamental error that can occur in the communication system like this.

In any situation where there is a failure of the two devices to communicate via a serial data link, the first place to look for a problem is in a wiring error.

Note that in implementations where there is a USB-to-serial adaptor integrated into the cable assembly, it will be impossible to test for wiring error with a simple continuity check of the cable, since the cable at the E-TEC EMM connector has no continuity to the USB connector. More sophisticated methods will have to be used to check for a wiring error. Generally anyone who will know how to make such a check won't need me to describe how to perform the check, so I will omit any description of it here.