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PF2100

Modbus Card Instruction Manual v1.1

Jan 5, 2012

Page 15

Testing the Modbus Card

There are a many different ways that you can determine if the Modbus Card is functioning correctly. These should generally be tried in this order:

1. Try to read a register from the PF2100 using the Modbus Master Device on the bus

2. Look at the LEDs on the Modbus Card

3. Use a PC Software Based Modbus Test Program

4. Use a Network Analyzer, Logic Analyzer, or Oscilloscope

Using a Modbus Master

First ensure that the Modbus Master Device that is intended to be used with your final system installation is setup correctly on the same RS-485 bus as the

Modbus Card. Then attempt to read a register from the PF2100 over the Modbus link. Consult “Appendix D – Troubleshooting” to determine which register

address(es) that you want to read and the meaning of the result(s) that you might get back. If the value that comes back is what you expect, then your card and

bus are probably setup correctly.

Using the LEDs

If your Modbus Card is powered up, you should see the “RUN” LED blinking at a fixed rate of about 2Hz. In addition, the “RX” LED will blink once briefly every

time that the Modbus Card sees a voltage transition on the bus. The “RX” LED will blink regardless of whether the packet was actually addressed to the Modbus

Card or even had a valid CRC. The “RX” LED is therefore a useful tool to visually see the total amount of traffic (or noise) on the bus. If a successfully received

packet was addressed to the Modbus Card, the “TX” LED will also light up almost at the same time as the “RX” LED indicating that a response has been

transmitted. If some of these LEDs are not behaving as described above, then consult the troubleshooting section of this document for further advice.

Using a PC Software Based Modbus Test Program

If you cannot figure out what is wrong with your Modbus Card by using the intended Master Device or by looking at the LEDs, you may want to use a laptop with

an RS-485 dongle and some test software to attempt to communicate with the Modbus Card at other locations along your bus. Some examples include:

• The “SeaLINK+485-DB9” available from “Sealevel Systems Inc.” is a USB to RS-485 dongle for Windows PCs.

• “Simply Modbus Master” and “Simply Modbus Slave” are test programs for Windows PCs available from a company called “Simply Modbus” in both paid

and trial versions.

Using a Network Analyzer, Logic Analyzer, or Oscilloscope

A portable Network Analyzer, Logic Analyzer, or Oscilloscope are useful for checking timing, level, and noise issues that might be present on your bus. Be sure to

verify that the Master Device is putting out signals on the bus that are compliant with the specifications for the Modbus Card listed on page 4.

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