Modbus vs. Modbus RTU 2

[gcaudill]

I have two devices:
Device #1 - Honeywell UDC3300 with Modbus RTU protocol
Device #2 - Newport iSeries with Modbus protocol

Are these two devices compatible on the same network? Is "Modbus RTU" different from "Modbus". Both use the RS-485 physical layer.

 

[djs]

Hello;
Modbus and Modbus RTU are really two different protocols. These two devices cannot exist on the same network.

Modbus transmits all information in ASCII hex characters. Modbus RTU transmits all of the data as binary information. Also the way the error checking checksum is calculated is different for each type.

Both of these protocols are master-slave with only one master allowed on each network. So you can not have one Modbus and one Modbus RTU master.

 

[gcaudill]

Thanks djs for the clear answer.

 

[TransmitterTamer]

Some newer Modbus devices sense which version of Modbus you (as bus Master) are using by examining the start of message flag, and they change their operating mode to reply to you in the same mode you used. I have seen this recently in some electrical power meters.

If you are trying to get a Modbus network running, you probably will find the ASCII version of Modbus much easier to debug, but the ASCII transmission speed is slower, and less robust in the presence of electrical noise. Once you know your network is running in ASCII, you can change to RTU, and your network should continue to work. The RTU (binary) version of Modbus runs much faster, and survives noise a bit better as well. So check your Modbus devices to see if they can handle both ASCII and RTU. I think many of the newer devices will work with both versions.

Remember that Modbus is a Master-Slave system. The Master speaks, and only the Slave who just heard his/her name has permission to speak. If you address a non-existant device, you will not see anything happen on your network. Second, be sure each device has a unique address, because otherwise you will have data collisions, and Modbus does not know how to handle collisions. I suggest you test each device and make sure you know what it's address is before you attempt to connect it to a Modbus network, because devices from a given manufacturer will probably all come with the same (default) address. You probably will have to change the bus address of every device before you connect any of them to a network of other Modbus devices.

Two more things; First, Modbus can be either 2 wire or 4 wire. The 2 wire version is half-duplex, and requires an adapter that can switch between receive and transmit modes automatically if you want to use a PC as a Master. The RS-232 Request to send line is used to control which way the adapter is translating. The 4 wire version is full-duplex, and much different because the Master speaks on one pair of wires, while all of the Slaves speak on the other pair.

In addition, Modbus devices can have fast rise and fall times, so you need to use a twisted pair (or two) cable, and make sure the pair(s) are properly terminated in 110 or 120 Ohms. You may also need to add what are called Bias resistors. Cat5 cable works well for Modbus, but if you are runing half-duplex, 3 of the 4 pairs in the cable will not be used. Even if you run full duplex Modbus, you will still have 2 pairs left unused in Cat5 cable. However, you probably can find a way to use those remaining pairs for other purposes. That purpose could be another Modbus network, parallel to the first, but with another group of Modbus devices.

Check the B and B website at

http://www.bb-elec.com

B and B has a lot of good information about Modbus that you can download free.

 

[GroundGuru]

I cannot imagine a Modbus device design only supporting ASCII, it is the nature of the beast to be binary (RTU) first and maybe ASCII no matter how it is documented in Sales literature. ASCII protocol went out of vogue when the TTY died...IMHO.

 

[pchipkin]

If you ever need to debug a Modbus message stream to understabd why you are not getting the data you expect then you could use the free Modbus message parser that is available at

http://www.chipkin.com/a/resources/modbus/parser