ModbusASCII versu Modbus+

[russrmartin]

In all my years of control work, I've had limited exposure to Modbus communications. Can anyone explain to me the difference between ModbusASCII and Modbus+? Specifically, what are the differences between network architecture, cabling, etc? How do the two networks look different from one another with regards to hardware used? I found the Modbus.org website to be a poor resource for answering this question.

 

[gclshortt]

http://modbus.org/docs/PI_MBUS_300.pdf

The above reference does a good job in explaining.
Note: The Modbus TCP is similar to RTU except that you are using an Ethernet connection.

Hope this helps,
Garry
http://www.accautomation.ca

 

[bernie_carlton]

Modbus ASCII and Modbus RTU are both defined (and open) serial protocols for Modbus. Modbus+ is a closed proprietary protocol owned by Modicon.

Modbus TCP can be thought of as Modbus RTU with an Ethernet wrapper.

 

[danw]

In terms of performance:

The network architecture is
- RTU/ASCII over RS-232: one master, one slave; point-to-point

- RTU/ASCII over RS-485: one master, multiple slaves, sequential polling by the master; daisy chain

- TCP over ethernet: Standard ethernet hub and spoke or star architecture.

Modbus ASCII is capable of only half the throughput of Modbus RTU because it sends twice as much data as the RTU format, but ASCII is less susceptible to timing issues that are inherent in RTU. Telephone modem communications used the ASCII extensively. The drop off in phone modem comm has seen a decrease in the use of Modbus ASCII.

Neither RTU nor ASCII on a multidrop RS-485 network can have more than one master, unless a specialized black box 'bus moderator' is employed. The work-around is typically multiple serial ports.

Modbus TCP can theoretically handle more than one master/client, however multiple masters bombarding a low-end slave running an 8 bit micro with requests/queries is not likely to be successful.

Modbus Plus is licensed and relatively expensive. As far as I can tell, it's primarily used for CPU to CPU comm, not field device to CPU comm. In fact, I'm not sure I've ever seen a field device that runs Modbus Plus.

 

[marcius]

Modbus plus is token ring protocol. Device can send data when it has token. Token passed to each device in turn.
Modbus plus is implemented using chipset which available to schneider partners.
It uses propietary cables, terminators and taps.

 

[Pudden]

Hi,

We (HMS) have some basic information posted at:
http://www.anybus.com/technologies/modbusplus.shtml

BR

Patrick L

 

[BobB]

Modbus Plus is smart then - I have been using Omron Controller Link over a twisted pair and also fibre for years - it is token ring based. Good to see Schneider have woken up at last! Pain with proprietary cables, taps and the like though - but I bet it is pretty quick! Getting to really hate TCP/IP to be honest.