Loopback Testing for UDS1100 in RS485 4-wire or 2-wire Mode

How can I test the RS485 4 wire pins with loopback connectors on my UDS1100? Can I do this for RS485 2 Wire, also?


Loopback testing on a UDS1100 in RS485-4wire mode requires that Tx+ (pin 14) is jumpered to Rx+ (pin 21) and Tx- (pin 15) is jumpered to Rx- (pin 22).

Fig1: Loopback Connectors on UDS1100 DB25F DCE in RS485-4w/RS422 mode

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With the loopback connectors in place:

  • Ping the UDS and verify that it responds.
  • Launch a Hyperterminal session, setting it up to communicate over Winsock (TCP/IP) directed to the UDS's ip address/port.
  • In the Hyperterminal session, type a series of characters and verify that what you are typing is what appears in the Hyperterminal screen.
  • Optionally, you can send a text file that consists of known characters by clicking 'Transfer' -> 'Send Text File…'

To properly test the RS485 4 wire pins with a loopback on a UDS1100, disconnect both (+) and (-) wires from the terminals to verify that the loopback is or is not working.

When the loopback wires are connected you will see the characters you type displayed in the terminal window. When the loopback wires are disconnected, no data gets returned or displayed on the terminal session when data is typed or sent into the session.

It the echoed data is corrupt, garbled, or missing then there may be an issue with the device server's serial port.

If the data flow does not stop when the RS485-4w loopback connectors are disconnected from the UDS's serial port then there may be an issue with the network connection, i.e. a telnet client may be connecting to a raw TCP listener.

This loopback test is possible only for UDS1100 set to RS485-4wire/RS422 mode.

An option that successfully tests both RS485-4wire and -2wire modes is to use a UDS1100 connected serially to a second (known good) UDS1100.

Configure two UDS1100 units with the same RS485 mode settings and connect them serial to serial (i.e. they are connected to each other serially in identical 485 modes).

Make separate TCP socket connections to each of the two UDS1100s and pass data from one open socket connection to the other and vice versa. This will be a valid alternative procedure to verify the integrity of data being sent over the network to the UDS1100 being tested.

Fig2: Loopback Test vs Alternative Test with Second (known-good) UDS1100

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Fig3: RS485 Interconnections between UDS1100 Serial Device Servers

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[Originally Published On: 10/07/2010 03:33 PM]