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:
- Switch 1 – DOWN (Run Mode), UP (Setup Mode, Modbus/DF1 OFF)
- 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 24VDC 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 24VDC 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 24VDC 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: