As far as I understand the Modbus protocol in OSI level 2 allows Modbus messages to be running at 9600bps or 19.2Kbps at maximum.

By converting Modbus frame into Modbus/TCP would not actually increase its data transferring speed, am I wrong?

My theory is very simple, but could be wrong, by putting a 4 cylinder car running on a street at 40mph into a super highway won’t make that car be capable of running at 100mph.

I am much appreciated if someone can explain to me and correct me.

Thanks.

 

Hello,

The speed of Modbus transmission is as fast as the slowest element in the chain. You can send modbus RTU frames as fast as your hardware work (it could be 1Mb/s) Of course if you have RS232 device and convert the frames to Modbus TCP/IP the device doesn't get faster transmission.

Andrzej
www.modbus.pl

 

I am reading your question and trying to decide what you are really asking.

Maybe this will simplify it. You will be getting as much information as you are asking for in a fast and timely manner. From your question, I think you are asking if it is akin to using an old MB to Ethernet bridge, in that the data comes out of the PLC at 19,200 and is put onto the Ethernet.

It is not like that, while I do not think that you will be filling up a 100mbit pipeline with PLC data, you are getting the higher speed communication that you are wanting.

I hope this helps.

 

Most implementations i have seen will let you select any baud rate for serial modbus. Disregarding the standard i guess. If you are using Ethernet (tcp/ip) speed is fixed at 10/100/1000 depending on your hardware... I guess a different modbus standard applies in this case.

 

Modbus/TCP will be sent at whatever speed your Ethernet connection is. That could be 10Mb (10 mega-bits per second), 100Mb, or even 1Gb (if the hardware supports that speed). Most Modbus/TCP hardware these days is 100Mb, although a few older devices are 10Mb. At those speeds, the bottleneck tends to be how fast the ends of the connection can process the messages.

However, if you are using a serial to Ethernet converter then the bottleneck will be the serial link and you will be limited to whatever speed that runs at.

 

Both Modbus/TCP and Modbus/RTU can be sent via an media which you have devices for - so you can send Modbus/RTU encapsulated in TCP/IP or UDP/IP at 1Gbps or even 1 gazillion-billion bps.

The big difference (& problem) with Modbus/RTU is that it lacks a sequence number so when pushing Modbus/RTU for max performance with short time-outs etc, you risk mismatching responses and requests (and no, the TCP sequence number does NOT help!)

For sure, 'bridging' Ethernet Modbus/TCP to a serial Modbus/RTU device won't speed it up. People buy bridges for this, and although it offers 'multi-master' sharing, it at best slows down the comms.

 

M Griffin is right.

Your theory is wrong because it's based on a wrong analogy. Modbus/TCP is not like "putting a 4 cylinder car running on a street at 40mph into a super highway", but rather like "putting a 4 cylinder car inside an airplane". No matter how many cylinders the car has, it will fly.

 

If you are talking about converting Modbus RTU (serial) to Modbus TCP (using a gateway), then yes, the fastest speed will be that of your slowest point in the network.

Best regards,

Gonzalo Jimenez
http://www.linkedin.com/in/gonzalojimenezyamasaki
http://sites.google.com/site/gonzalojimenezyamasaki/