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Modbus RTU 2-wire Master with 4-wire Slaves
Need help connecting a 2-wire Modbus RTU master (Honeywell Eagle) to 4-wire slaves (Mitsubishi FR-D700 series inverter), with the wiring or the requirement of a converter device.

Hello,

I would be grateful for any help with connecting 4-wire Modbus RTU slaves to a 2-wire master controller. The controller is Honeywell Eagle CLEA2000B31, with Modbus RS-485-1 connectors labelled simply + and -. The slave devices would be Mitsubishi FR-D700 series inverters, equipped with RJ45 connector.

The controllers' (CLEA2000B31) communication manual includes diagrams of connecting 2-wire slaves to the bus, but no mention of 4-wire connections. The manual is located here:

http://products.centraline.com/en/pdf/eagle-comm-en2z1002-ge51r0117.pdf

The inverters' (FR-D700 series) manual states the bus would be "Conforming standard: EIA-485 (RS-485)" and "Transmission form: Multidrop link"

The inverter manual is located here (a large document with 300+ pages):

http://dl.mitsubishielectric.com/dl/fa/document/manual/inv/ib0600366eng/ib0600366engg.pdf

Manual page 181 (.pdf file page number or page 179 as "printed") lists the RJ45 connector pin numbering, with pins 3-6 being relevant. From modbus.org specs I understood that for 2-wire communication pins 4 and 5 are used.

I thought this kind of connection would require a converter device, but the inverter manual page 183 does include the following detail:

"Two-wire type connection -
If the computer is 2-wire type, a connection from the inverter can be changed to 2-wire type by passing wires across reception terminals and transmission terminals of the PU connector pin."

So the wires SDA and RDA should be connected, and SDB with RDB. But does anyone here have experience with Honeywell controller and 4-wire Modbus devices, is a converter required? And if so, is there a recommendation for a DIN-rail mounted one?

1 out of 1 members thought this post was helpful...

No experience with your stuff, but no converter is needed. In fact 2-to-4 wire converters are not even available commercially, because all the other 4 wire RS-485 Modbus devices run as 2 wire devices when the same polarity send/receive driver lines are jumpered just as you state: SDA and RDA should be connected, and SDB with RDB.

There are commercial RJ45-to-screw terminal adapters, so that you can make jumper connections.

Try Googling RJ45 plug to screw terminal

Thank you for your response. Initially, when I researched the RJ45 connector, I was happy to find out there was in fact a screw terminal adapter. But on the inverter, the space is tight on the RJ45 connector, so it cannot be used. So it'll be a bit of a patch cable and the wire connections need to be made elsewhere.

In fact, it's the first time I've encountered a 4-wire Modbus RTU device (and especially with RJ45 connector). I was thinking that since 4-wire is full-duplex operation, the device is expecting the communication to be certain way, so is the half-duplex 2-wire device. Is there a chance that there could be communication issues, since Honeywell documentation doesn't even mention 4-wire bus devices?

I'm asking since there'll be a good while until we could actually power on the devices and do any testing. But there's plenty of time for making the wiring beforehand. And if there's no need for a converter, even better. :)

1 out of 1 members thought this post was helpful...

Hardware-wise, 4 wire RS-485 is capable of operating full duplex, but by jumpering a half duplex port is created.

The slave drive does not need full duplex functionality (the ability to send and receive at the same time).

The sequence of Modbus event is sequential (one following another) over time: the master sends, the drive listens, the drive sends a reply, the master listens, all in sequence.

So all that is required is half duplex, either sending or receiving, but not both at the same time.

Full duplex offers no working advantage, and probably is not 'implemented' in the firmware, given that it is a Modbus port.