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I've struggled with what is the proper bit order for Modbus serial communication. I took a look at the "MODBUS Protocol Reference Guide" (PI-MBUS-300 Rev. J) and noted that the proper method appears to be LSB to MSB (Left to Right Reading).

I often use MODSCAN and noted that it seems to be MSB to LSB (Right to Left Reading) and there doesn't seem to be an option to reverse this.

I'm not a serial expert, but I thought that serial information (not specifically MODBUS) typically transmits the LSB first, which to my mind seemed to explain why MODSCAN used MSB to LSB (Right to Left Reading).

Is this assumption correct?

I also seem to remember that several HMI programs I've worked with have an option to define who the software should be looking at the MODBUS information. When I use MODSCAN to read MODBUS information, some equipment seems to transmit the MSB first and other equipment seems to transmit the LSB first.

From a MODBUS Purist point of view, should the equipment be transmitting the MSB first?

I linked a Rosemount DCS to Modicon PLC using the DCS's Modbus link a while back, mapping bits in one system to the other. When I fired it up all my alarms bits were backwards and I spent days re-typing all the messages.

I can't recall if Modicon word is LSB to the left though.

I think most users of Modbus ignore the book and change it to suit themselves.

Roy

Hi,

Please read the "How Characters are Transmitted Serially" part of this web page:
http://interface.centraltreasure.com/modbus_protocol.html

I would associate an MSB to LSB (most significant bit to least significant bit) to transmit the "leftmost" (most significant bit) first, so I'm confused by your saying "Right to Left Reading"

According to
http://doc.seg-pp.com/doku.pdf/sl/kommunikation/modbus/modbus_e.pdf
The least significant Bit is sent and received first

I'd call this LSB to MSB transmission - what do you think?

I think the problem is in naming the bits, and not was is sent first.

I have seen 3 kinds of naming bits.

1. Usually naming bits in computers is 0=LSB 15=MSB.
2. In old Modicon PLCs 1=MSB 16=LSB.
3. I have seen HMIs that use following naming: 1=LSB 16=MSB

Robert

Ya, you are right!

There is another good source of information at http://www.simplymodbus.ca/FAQ.htm