Modbus 2 wire to 4 wire converter
Monday, December 25, 2017
the world's leading automation protocol.
Why somebody use 4 wire network for Modbus RTU.
Modbus is not full duplex so 2 wire should be ok!!
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.
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.
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.
Sunlux Technologies Ltd.
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
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.
Lynn August Linse,
Robust DataComm Pte Ltd (www.robustdc.com)
The wiring diagram shown is obviousy for 4-wire communication. If all your devices support 4-wire then goahead 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.
Sunlux Technologies Ltd.
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Modbus 2 wire to 4 wire converter
Monday, December 25, 2017
the world's leading automation protocol.
Communicating between "boxes" using Modbus. Master is a 4-wire system and Slave is a 2-wire system. What is recommended cabling? What is recommended termination resistor?
connect A signals and call them +
connect B signals and call them -
One question on the slave device, is this device configurable for RS485 2-wire & 4-wire system? If it does, then I would suggest to implement 4-wire full dulpex system.
As for the communication line termination resistor, usually is 120ohm to match typical communication cable impedance. Each resistor has to be installed only at the end of the Receiver cable pair (Rx+ & Rx-) on Slave device and (Rx+ & Rx-) on Master device.
I'm having a similar problem, except the master device is half-duplex and the slave is full-duplex. I tried this wiring scheme, but when the slave device responds, an echo occurs (the response is sent back to the slave as well as the master.) Any way to prevent this behavior?
Yes, a kind of "pot luck" problem. Some 4-wire produces turn off their receiver during transmit and thus you can just short the pairs togather and make them 2-wire. Others (as you've found) do not and thus see an echo. Common symptom is Masters is it sends only 1 byte of a request, then "shuts up" since it sees itself talk and thinks the RS-485 network is in fault.
When echo causes problem, your only solution will be to use a piece of hardware (a 2-to-4 wire or RS-422 to 485 converter) that prevents an echo of 4-wire data received back on the other pair of the 4-wire. http://www.robustdc.com and others sell such products.
I'm having a similar problem, except the master device is half-duplex and the slave is full-duplex. I tried this wiring scheme, but when the slave device responds, an echo occurs (the response is sent back to the slave as well as the master.) Any way to prevent this behavior?
Living in the complex world of the future is somewhat like having bees
live in your head. But, there they are.
You have clicked on the "?" button for search help. To search the site, enter your search terms in the box labeled "search the site" and hit Enter.
Some tips for better search results.
- Precede each search term with a "+", as follows:
- +Modbus +TCP
- Use double quotes around phrases, as follows:
- +"Allen Bradley" +ethernet
- To exclude a word, precede it with a "-", as follows:
- +Modbus -Plus
- Note that common words (and, that, etc.) and words shorter than 2 characters are automatically excluded from searches.
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Modbus 2 wire to 4 wire converter
Monday, December 25, 2017
community of automation professionals.
Why somebody use 4 wire network for Modbus RTU.
Modbus is not full duplex so 2 wire should be ok!!
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.
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.
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.
Sunlux Technologies Ltd.
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
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.
Lynn August Linse,
Robust DataComm Pte Ltd (www.robustdc.com)
The wiring diagram shown is obviousy for 4-wire communication. If all your devices support 4-wire then goahead 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.
Sunlux Technologies Ltd.
Quantum Mechanics is God's version of "Trust me."
You have clicked on the "?" button for search help. To search the site, enter your search terms in the box labeled "search the site" and hit Enter.
Some tips for better search results.
- Precede each search term with a "+", as follows:
- +Modbus +TCP
- Use double quotes around phrases, as follows:
- +"Allen Bradley" +ethernet
- To exclude a word, precede it with a "-", as follows:
- +Modbus -Plus
- Note that common words (and, that, etc.) and words shorter than 2 characters are automatically excluded from searches.
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If you're not already a member, consider joining. It's free, and you can customize the content you view, as well as being able to subscribe to threads and topics, getting new posts delivered to your email as they appear.
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If you're not already a member, consider joining. It's free, and you can customize the content you view, as well as being able to subscribe to threads and topics, getting new posts delivered to your email as they appear.