About RS485, LonTalk and Metering


Thread Starter



I began working a week ago, and now I have problems with a Big Project. It is about metering every cost center of a big refinery. The problem is that we should use our equipment (with Modbus RTU protocol over an RS485 network) to get the data, there is a big distance between every center and the Central Unit, and more than 100 meters. We are considering the fact of use repeaters (for RS485) and probably an optic fiber line to the longer distances. My question is: Is it possible to connect twisted pair, optic fiber and a lot of meters in a RS485 network? We have another line of meters that accept LonTalk protocol, Should we change our network to get more reliable data (what about the costs)?
And, what about the distance and the amount of data transmitted? Please I need comments.

Robert Willis


Have you considered using Ethernet as your backbone network and then using the required Ethernet to Modbus Bridges?

Schneider Electric does this with their PowerLogic (Power Metering), and PLC's. Check out "www.powerlogic.com":http://www.powerlogic.com for information on their Ethertnet to Modbus Gateways (EGX200 and EGX400). Both of these devices support the Modbus RTU serial and Modbus TCP/IP protocols.

If you can provide additional information on the manufacture of your current power merters I can help you develop a network topology diagram that will allow you to integrate the equipment.

Robert Willis
Square D Company
Houston, Texas
E-Mail: [email protected]

Cameron Haegle

You not need any repeaters on the RS-485 lines. RS-485, by specification can go out 2000 meters (if memory serves). That is 2000 meters from one end to the other, crossing all connected drops.

The differential nature of RS-485 makes it less susceptible to external interference and good for very long cable runs.


Kinner, Russ

I've just finished a project using RS485 (Modbus protocol) talking to 16 tank gauges at a pipeline storage facility. Distances are in the 400-600
meter range from the control room to each grouping of 5-6 tanks (50-200 ft. diameter and all 50 ft. tall) and then its about 500 meters between all the tanks of each group.

We installed repeaters at each tank group and at the control room from a common PLC serial port to keep the total cable length down but there is no
problem using a single line for 600 meters. Cabling was by overhead pole so each tank's cable was routed from the top of the tank to ground level, routed a short distance to the closest pole and then run back up about 10 meters before heading to the next location.

Other than some problem with lightning, it's been working for 6 months with no problem. If you are routed in a similar fashion (above ground), I would suggest some voltage protection devices on the RS485 lines. Several manufacturers supply protective devices (we used Phoenix Contact)
specifically designed for RS485.

Russ Kinner
AVCA Corporation
Maumee, OH