Different RS485 wiring on same series

[Gavin_Bryson]

Hi,

I'm retrieving metering data via Modbus TCP using an EGX Link 150.

I've got third party single phase meters in a RS485 series all connected to an EGX Link 150. The RS485 wiring for the meter is +ive, -ive and gnd.

However, a customer wants to introduce the 3-Phase Schneider iEM3155. This meter uses 4 terminals in its communication port, +ive,-ive,gnd/shd, 0 V. 

Is it possible to mix 2 different RS485 configurations on the same RS485 series, especially if 1 type doesn't use 0 V reference?

Additionally, the Modbus Communication parameters would be the same on both meter types, i.e. baud rate, parity and stop bits.

I realise this might not be the best place to ask the question but I've spent a few days online trying to find an answer with no success.

If anyone can offer any advice it would be greatly appreciated.

Kind regards,

Gavin

[Charles_Murison · Accepted solution]

Hello @Gavin_Bryson ,

In RS-485, there is a few different implementations that exist, some will use 3 wires where the common point is connected to the grounding/shielding point and other devices where this is 2 separate points. While I cannot speak for the iEM3155, EGX150 or 3rd party meters the PM8000 also has 4 wire configuration and has the following in the install guide where the common is connected to ground. 

Regards,

Charles

[Gavin_Bryson]

Brilliant,  thanks @Charles_Murison , I will take a look at the PM8000 document.

[jmbruges]

Hi @Charles_Murison,

I found very useful your post.

I have some questions regarding wiring IEM3255 (modbus interface) in a daisy chained configuration.

I will be wiring about 11 energy meters to a Siemens PLC, which will act as a master and the meters as slaves.

All IEM3255 come with D+, D-, Shield and 0V connectors. The Modbus connector that interface the PLC and the modbus topolgy only has 3 wires possibilities as in Figure 1 (Communication x2 and the shield connector which is done by chasis wiring to the shielded cable)

Figure. CM1241 wiring.Figure 1.

In the above topology, the first node in the Modbus is the PLC connector, which is the master in the communication. I used the bias configuration of the connector that uses pin 5 with a 390 ohms resistor. In the CM1241 documentation, pin 5 can be used as a logic ground, but since it is required to start (or end) the topology with a bias interface, the connector to the CM1241 is then used as bias connector. On the other hand, the Profibus connector has no possibility to plug descently to the pin 5 of the male connector. I don't know if there are modbus connectors with this possibilities but this connector is the one provided by siemens module.

@Charles_Murison What would you recommend to wire this topology where the master is not able to connect to a 0 volt reference? Should I follow your previous post where only 3 wire configuration was stablished between the slaves (communication x2 + Shield in a daisy chained)?

Thanks

[Charles_Murison]

Hello @jmbruges ,

I cannot give you a recommendation as I am not familiar with the IEM series of meters nor this PLC. 

Regards,

Charles

[Grant_IPACS]

@jmbruges depending on the cabling, distance, etc determines biasing & terminating requirements...

i suggest trying without them first.

[jmbruges]

Thank you for replying @Grant_IPACS  and @Charles_Murison.

The project I am working on consist of two metering systems (each system consist of a series of energy meters connected to a PLC).

One of the systems contains 11 IEM3255 connected to the PLC under a relative short distance, i.e. all the equipment is placed at the same cabinet. The other system the PLC is on a different location than the slaves and the slaves are scattered at the factory.

Yesterday I found this other topic: PM5300 vs 5560 - 3-wire RS485 wiring differences? 

The power logic have the same connections (4 wire) than the IEM3255. 

I think James Clark has an asnswer to what I am looking for. I think the 0V reference in the IEM3255 should be used in a 3 wire configuration between slaves and connect to the chasis of the PLC connector.

Thanks once more.

Javier