Modbus RTU 4 or 2 Wire RS485

B

Thread Starter

Brian

Why somebody use 4 wire network for Modbus RTU.
Modbus is not full duplex so 2 wire should be ok!!
Any advantages?

Thanks,
Brian
 
J

Jerry Miille

A 4 wire hookup is much simpler to troubleshoot and to use. The master transmitts to all the slaves and all the slaves transmitters are connected to the masters receiver. The slaves only "hear" messages from the master and the master can only hear the slaves responses.

With a 2 wire hookup, everyone hears all the traffic. Slaves receive not only polls from the master but also can hear every response from all other slaves. Unless the hardware specifically disables it, everyone also can hear themselves talking. If you have an interrupt per character type system, a 2
wire system will significantly add to the interrupt overhead.

With that said, Modbus will indeed work on a 2 wire system but some manufacturers equipment may not.

Jerry Miille
 
The only difference really is that echo suppression has to be handled properly in all of the devices on the 2 wire network. 4 wire has 2 sets of differentials and does not have to worry about suppressing the echo. I've dealt with many headaches on echo suppression over the years but it seems to be fading. 4 wire RS485 and RS422 has always been more robust for me.
 
G

Ganesh Okade

Brian:
4-Wire communication is differential and hence any noise on the bus lines cancel each other unlike in the case of two wire which makes it more noise resistant. Hence 4-wire is still used - it has nothing to do with duplex communication.

Ganesh Okade
Sunlux Technologies Ltd.
www.sunlux-india.com
 
I would also be interested in this. I am doing Modbus for the first time. I have 6 PLC's each with a Sinec L2 and am trying to conenct to a PC with an RS232/485 adaptor. Wiring diagram shows 4 wires (and earth) - do I need this.

Also does anyone have a low cost SCADA package for Modbus RTU - I only need to see about 10 registers in each PLC

Bye
 
L

Lynn at Alist

See http://www.robustdc.com/library/appnotes/san005.shtml for some explanation of RS-485 and grounding issues.

You need to handle the earth for RS-485 signal reference and current return somehow - either you use the 5th signal wire or all 6 of your PLC must reference the RS-485 to the same physical ground. Since you use RS232/485, it will depend on the converters you buy. In a lab, RS-485 4-wire without concern for grounding works fine, but systems that don't use the 5th signal ground wire when it should be used will have long-term robustness problems.

Best Regards

Lynn August Linse,
[email protected]
Technical Director
Robust DataComm Pte Ltd (www.robustdc.com)
 
G

Ganesh Okade

Hello Adrian,
The wiring diagram shown is obviousy for 4-wire communication. If all your devices support 4-wire then go ahead and use it - its has better noise immunity and you need not worry about echo as others have noted. If not convert the 4-wire system to 2-wire and use it in 2-wire mode.

What is your budget for the SCADA package. Are you looking for a single screen which displays the 10 registers and maybe log them?

Ganesh Okade
Sunlux Technologies Ltd.
www.sunlux-india.com
 
The only difference really is that echo suppression has to be handled properly in all of the devices on the 2 wire network. 4 wire has 2 sets of differentials and does not have to worry about suppressing the echo. I've dealt with many headaches on echo suppression over the years but it seems to be fading. 4 wire RS485 and RS422 has always been more robust for me.
interesting reply
can you provide more info please how do you tune echo? do you use osciloscope?
 
A 4 wire hookup is much simpler to troubleshoot and to use. The master transmitts to all the slaves and all the slaves transmitters are connected to the masters receiver. The slaves only "hear" messages from the master and the master can only hear the slaves responses.

With a 2 wire hookup, everyone hears all the traffic. Slaves receive not only polls from the master but also can hear every response from all other slaves. Unless the hardware specifically disables it, everyone also can hear themselves talking. If you have an interrupt per character type system, a 2
wire system will significantly add to the interrupt overhead.

With that said, Modbus will indeed work on a 2 wire system but some manufacturers equipment may not.

Jerry Miille
interesting reply - any chance you provide more details on benefits of 4 wire- if slaves can hear other slaves what us the issue with that? they all have different addresses but maybe it makes comms faster because slaves dont need to processs other slaves traffic?
 
This thread is from over 20 years ago. Chances are, the members who posted back then no longer frequent this forum and you're not very likely to get a response from them.

But I'll try to answer your questions.

can you provide more info please how do you tune echo? do you use osciloscope?
You don't do any tuning for this as a user. The device manufacturer has to have designed their hardware, or written their software, in such a way that allows the device to ignore its own transmissions (i.e. disable its receiver during transmission). If the device has not been designed this way, it's going to receive its own transmissions. The 4-wire RS-485 device may end up in an endless loop when wired on a 2-wire network, replying to its own transmissions (for example, a Modbus write single register packet is the identical for both the request and response).

any chance you provide more details on benefits of 4 wire- if slaves can hear other slaves what us the issue with that? they all have different addresses but maybe it makes comms faster because slaves dont need to processs other slaves traffic?
Maybe 20 years ago this made a slight difference, but today's devices are much faster and communications takes an eternity compared to the speed of the CPU, so some extra interrupts are not going to bog down the device. But your understanding is correct. If the slaves are not receiving other slaves' responses, then their serial drivers are not burdened by the overhead of processing the additional traffic.
 
This thread is from over 20 years ago. Chances are, the members who posted back then no longer frequent this forum and you're not very likely to get a response from them.

But I'll try to answer your questions.


You don't do any tuning for this as a user. The device manufacturer has to have designed their hardware, or written their software, in such a way that allows the device to ignore its own transmissions (i.e. disable its receiver during transmission). If the device has not been designed this way, it's going to receive its own transmissions. The 4-wire RS-485 device may end up in an endless loop when wired on a 2-wire network, replying to its own transmissions (for example, a Modbus write single register packet is the identical for both the request and response).


Maybe 20 years ago this made a slight difference, but today's devices are much faster and communications takes an eternity compared to the speed of the CPU, so some extra interrupts are not going to bog down the device. But your understanding is correct. If the slaves are not receiving other slaves' responses, then their serial drivers are not burdened by the overhead of processing the additional traffic.
thanks for reply
it makes sense that device should be capable of ignoring its own traffic

are there any reasons why in practice troubleshooting 4 wire modbus would be easier? what tools would you use - for ascii it would be putty client but what about rtu which is binary format?
 
are there any reasons why in practice troubleshooting 4 wire modbus would be easier? what tools would you use - for ascii it would be putty client but what about rtu which is binary format?
I actually think 4-wire networks are more difficult to troubleshoot, at least from an installation perspective. Instead of mixing up just 2-wires (+ and -) now you could mix up any combination of 4 wires (TX+, TX-, RX+, RX-). Perhaps from a software developer's standpoint, it's simpler to troubleshoot, since there are less messages that each slave's communication driver sees.

As far as tools to use, 4-wire vs 2-wire doesn't really impact this, unless you're trying to use an oscilloscope to measure the signals, since you would have twice the number of signals to look at with 4-wire. ASCII and RTU are two different versions of serial Modbus communication. Regardless of which Modbus version is used, software tools such as master simulators, slave simulators, and serial capture software are useful for troubleshooting Modbus communication issues. Some examples of these include the following (Disclaimer: I am not associated with, nor do I specifically endorse, the following. They are provided merely as examples.) :

WinTECH - ModScan/ModSim
https://www.win-tech.com/

Witte Software - Modbus Poll / Modbus Slave
https://www.modbustools.com/

Simply Modbus - Master Emulator / Slave Emulator / Client Emulator
https://simplymodbus.ca/

Electronic Team (formerly Eltima) - Modbus Sniffer

https://www.eltima.com/modbus-sniffer.html

Tibbo Technology - IO Ninja
https://ioninja.com/
 
Top