I have a serial connection from a Lantronix product to a serial device and it's not working. What are some steps I can take to troubleshoot the connection?

These are very general procedures that can be used to troubleshoot a serial connection to a Lantronix product.

Click here to go to the Lantronix Product Index, find your product and click on the orange "Docs and Firmware" link to find documentation for your product. Check the User Guide or Installation Guide for your Lantronix product for connector pinouts and other details.

For details of 3rd party serial devices please consult the documentation for the product or contact the manufacturer.

When troubleshooting a serial connection (or anything else) only make one change at a time, and document each change when you make it. That way it's easy to get back to where you started, if needed.

Troubleshooting steps:

Make sure that the Lantronix product and the serial device are both set to the same speed, character size, parity, stop bits and flow control method.
Try a different serial cable. If the new cable works, discard the old one.
Disable flow control on both the Lantronix product and the serial device. If you get communication without flow control, but no communication with flow control, make sure that both ends of the connection are using the same kind of flow control, or just don't use flow control.
Check that the Data Output pin of the Lantronix product is connected to the Data Input pin of your serial device, and that the Data Input pin of the Lantronix product is connected to the Data Output pin of your serial device. There must also be a connection from the Signal Ground pin(s) on the Lantronix product to the Signal Ground pin of the serial device.

Serial connectors come in many shapes and sizes, so check the documentation for your Lantronix product and for your serial device to find out what signals are on which pins. This is referred to as the connector's "pinout".

Remember: Some RS232 serial ports are DCE (Data Computing Equipment) ports. DCEs are usually labeled "backwards", i.e. TX is an input and RX is an output. Make sure you understand which pin(s) are outputs and which are inputs on both sides of the connection when you are designing the connecting cable. An example of a Lantronix product with a DCE connector is the UDS1100.
Make sure your serial cables are not excessively long, especially in electrically noisy environments. In general a serial cable should not be longer than 50' for communication at 9600bps. Faster speeds require shorter cables, slower speeds allow longer cables. In general, the shorter the cable and the slower the serial speed the more reliable the connection will be.
Try a different serial device of the same make and model. If the new one works, replace the original..
Try a different Lantronix product of the same model. If the new one works, replace the original.
On most Lantronix products you can connect directly to a serial port over a network connection. Connect a "loopback connector" that connects the Data Output Pin to the Data Input Pin, then open a network connection (e.g. telnet) to that port and type some data into it. The data should be echoed back to the network session. If you can't see what you're typing it's probable the Lantronix product's port is damaged or defective.
If you are replacing another type of connection (direct serial, dial-up, etc) with a network connection through the Lantronix product, hook the old connection back up to see if it still works. If not, resolve the problem with the original connection before returning to trying to get the network connection working.
Many 3rd party products come with a cable so they can be connected to a PC's serial Com port. If your product has such a cable, connect it to a PC and check to see if it works. If not the serial port on the 3rd party device may be defective.
Here is an article on RS232 serial cable wiring. Check that your cable is wired so that all inputs on one end are connected to the matching output on the other, and vice versa: RS232 Serial Cable Wiring

[Originally Published On: 03/29/2006 04:00 PM]

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