Re: Modbus Communication Question

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Comments:: 1) When PLC communicate with third party instrument, it needs to wait for the message to be completely sent out and then wait for the response from the other device. So the scan time will be longer because of the wait. You should not be communicating with third party device more often than necessary (e.g. use a clock pulse and send the command every 0.5 or 1.0 second) if you want to minimized the impact on the Scan time. : To capture input that are scan time sensitive, consider using the interrupt inputs which run independent of the scan time. Interrupt processing are handled first even during the communication. : 2) HCMOS output are logic level, and 1mA and drive a lot TTL or CMOS logic level inputs. It is not meant for driving heavy loads. It can drive a high efficency LED with 1mA output. : : Leon, : : How are you! : : I have already received your E-mail, And I have solved the problem. Thank you. : : I want to ask you two new questions now. First, I discovered that the PLC scan times : : become longer when the PLC communicate with the third device. why? Second, It seem : : that the drive capable of 16 HCOMS logic-level outputs on MD-EXP 4040 Module is smaller. : : It is only 1mA . That is to say the small current could not drive the most other device. : : For example, the LED,the Power Transistor, etc. : : Could you give me some suggest .Thank you again. : : best regards. : : Jackson : : 24/Jan/2002 : : : : The readmodbus command uses MODBUS read register (03) and write register (06) command internally. You only specify the slave ID, the MODBUS register address (0000 to FFFF, which maps to 40001 onwards). What are the expected command to read or write to Delta Inverter(VFD-M)? : : : READMODBUS and WRITEMODBUS commands are what we implemented in our programming language. When these commands are executed, the output from the PLC serial port will be MODBUS ASCII if the comm port number is 1 or 3, and the output is MODBUS RTU if the comm port number is 11 or 13. Instead of creating a separate command for RTU we decided that it would be easier to just use the COMM port number to distinguish between the two, since both ASCII and RTU are all called MODBUS protocols. : : : I hope these answers to your questions. : : : : Hello: Now I want to use a T100MD888+ PLC as Modbus RTU master to commnicate with Delta Inverter(VFD-M).But READMODBUS and WRITEMODBUS instruction look like not support The Modbus protocol.Because Modbus RTU's command format different : : : : from The READMODUS and WRITEMODBUS instruction format. : : : : I would hope get help. Thanks.

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