Application Note CTAN #165

Transcription

1 Application Note CTAN #165 The Application Note is pertinent to the Commander SE Family Interfacing Commander SE to Modbus Protocol Many customers attempt to connect foreign devices such as HMI displays and PLCs to the Commander SE using the built-in Modbus protocol support. If you re having trouble, here is a procedure that will help. It uses a readily-available ModScan32 program running on a PC to verify that the Commander SE and it s serial cable are working properly. Use a proper RS-485 Converter The Commander SE uses 2-wire RS-485 communications. Control Techniques sells a very convenient six-foot self-powered SE71 serial communications cable with the RS-485 converter molded into the cable. Users would be better served using a proven product like the SE71 rather than attempting to build a custom interface. This cable has a RJ45 telephone connector that plugs into the Commander SE and a 9-pin serial connector for the COM1 port on the PC. Set up the Commander SE for Modbus Three menu 0 parameters must be configured to establish Modbus communications. #00.41 = RTU Select Modbus RTU protocol #00.42 = 19.2 Select 19.2 kbaud #00.43 = 1.1 Set Commander SE serial address to 11 Get a Copy of the Modscan32 Program The West Virginia company WinTech makes a very nice Modbus utility called Modscan32. You can procure a free demo copy from their web site: This demo program will work for 30 days and each execution is limited to five minutes. Modscan32 acts as a Modbus Master and can be used to verify operation of the Modbus interface to the Commander SE.

2 Run the Modscan32 Program The Modscan32 executable is ModScan32.exe and may be easily started from the Windows START - RUN menu. The following display appears. First, we setup the specifications of what drive register we want to read over the Modbus network. Below we selected Setup Data Definition and then selected the drive serial address 11 (must match the Commander SE setting), read a holding register, ask for holding register (entered without the 4) and asked for a transfer of only 1 holding register. Holding Registers are specified as 4mmpp, so refers to Menu 0, parameter 2 (Max Speed in Hz). 2

3 Now we specify the display format using the Setup Display Options menu. We check Show Data and Unsigned Decimal. The option Show Traffic is very useful since it prints the transmitted and received bytes real-time. The serial interface is setup when you select connect on the Connection pull-down menu. Here we select a Direct Connection to COM1, baud to match the Commander SE setting, 8 data bits, no parity and 2 stop bits. The 2 Stop Bits surprise many customers, the Commander SE Modbus interface will not work with a single Stop Bit. 3

4 Click on Protocol Selections to select Modbus RTU. After clicking out of the Connection pull-down menu, the Main Screen will become active, with the data being read from the drive displayed and refreshed several times a second. Reading updates Real-time. The above setup executes the Modbus command Read Holding Registers, function code 03. You should be able to change the parameter value on the Commander SE drive and see it update on the ModScan32 display. 4

5 To write a value to the Commander SE, use the Setup Extended pull-down menu. First select the drive address, the register to be updated (here is shown as simply 2) and update only one point (register). Then a Preset Multiple Registers window will appear and you can type in a new value and transmit it by hitting the Update button. You should see the value change on the Commander SE drive. This operation is essentially Preset Multiple Registers, function code 16. If the previous two operations (read and write) operate successfully, then you can be assured that the Commander SE Modbus feature and the RS-485 cable are working properly. Questions?? Ask the Author: Author: Jim Lynch Jim.Lynch@emersonct.com (716)