RS485 Tips For MODBUS/RTU & DF1 Networks

The EtherMeter contains an RS485 port for Modbus/RTU and DF1 communications in multi-drop applications.

When using the RS485 port, the following EtherMeter terminals are used:

• 19  –  RS485A (-)
• 20  –  RS485B (+)
• 21  –  Signal GND Reference (contains current-limiting resistor)

In order to use RS485, attention should be given to the EtherMeter’s dip switch positions:

Latest EtherMeter:

• Switch 1  – DOWN, RS485 Mode, (UP = RS232 Mode)
• Switch 2  – UP, RS485 Mode, (DOWN = RS232 Mode)
• Switch 3  –  UP, Use 120 Ohm Terminator for RS485, (DOWN = No Terminator)
• Switch 4  –  N/C
• Switch 5  –  N/C
• Switch 6  –  N/C
• Switch 7  – UP (LCD Backlight ON), or DOWN (Power-Saver Mode)
• Switch 8  – DOWN (Run Mode), (UP = Setup Mode, Modbus/DF1 OFF)

Legacy EtherMeter:

• Switch 1  –  DOWN, Run Mode, Modbus/DF1=ON, (UP = Setup Mode)
• Switch 2  –  DOWN, RS485 Mode, (UP = RS232 Mode)
• Switch 3  –  UP, Use 120 Ohm Terminator for RS485, (DOWN = No Terminator)
• Switch 4  –  UP (LCD Backlight ON), or DOWN (Power-Saver Mode)

In my experience, it is preferable to use a single, centralized 24V_DC power supply to power the Modbus Master — along with all the remote RS485/Modbus Slave devices.  This ensures that signaling between all devices are referenced to a common ground potential.  In order to most-easily achieve this, a single jacketed cable can be used to transmit both the 24V_DC power and the RS485 Modbus signals.

For the most demanding (higher baud rate and/or long distance) applications, I generally recommend Belden 3084A cable, which contains two shielded/twisted pairs plus an overall copper braid.  The 24 gauge pair (Blue/White) has a characteristic impedance of 120 ohms, which is optimal for RS485 transmission.  The 22 gauge pair (Red/Black) is designated for 24V_DC and GND.  The cost is generally around $1/ft.  And for especially long cable runs, a more stout version is available, the Belden 3082A cable, whose power and data pairs are 15 gauge and 18 gauge respectively.  The cost is generally around $3/ft.

For more economical 2-pair wiring, I recommend Belden 8723 cable.  It consists of two 22 gauge shielded/twisted pairs (red/black and green/white).  Although it has a characteristic impedance of 52 ohms and therefore does not conform exactly to the RS485 specification, it can work very well for  shorter distances and/or lower baud rates.  The cost is generally around $0.40/ft.

In situations where it is not feasible to power the Modbus master and all remote slave devices from a single DC power supply, then all devices should be powered locally and individually using isolated DC power supplies.  Furthermore, the signal grounds of all devices should be tied together to a common voltage reference.  Each reference tie should utilize a resistor to prevent large currents from traveling on the reference wire.  For this purpose, Terminal 22 on the EtherMeter functions as a signal reference point with a built-in 100 ohm current-limiting resistor.

I highly recommend that anyone using Modbus/RTU over RS485 should browse the web for further information.  Here are a few articles that I’ve found to be particularly useful and interesting:

• B&B Electronics – Tips For Success In Using RS485 Data Communications
• B&B Electronics – The RS485 Application “Cheat Sheet”
• B&B Electronics – Troubleshooting Guide For RS485
• Chipkin Automation Systems – RS485 Cables – Why you need 3 wires for 2 (two) wire RS485
• RS485 – Wikipedia