The differences are minor, but worth mentioning.īy the way, you should separate RS-485/RS-422 from Modbus RTU/ASCII. You should be aware though, speaking from an industrial equipment standpoint, there is "Modbus"-Modbus, and Modicon Modbus (or Modbus+).
#MODBUS RTU WIRING FREE#
If you want to read them, the Modbus Implementation and Specifications are free on the internet. I've seen industrial devices that don't support more than 32 slaves on one RS-485 network. In reality though, devices may choose to use smaller/more power efficient/cheaper chips that only support 32 slaves (or less). A Modbus device that complies with the spec is supposed to be able to be a part of a network of up to 246 slaves, so you should only be seeing devices that support larger networks. The rest is up to the hardware specification by the device vendor. Shared global database that can be updated with each token rotation. When Modbus protocol is used on higher level networks, broadcasts may not beĪllowed or may be replaced by other methods. When the slave sends its response, it places its own address in thisĪddress field of the response to let the master know which slave is responding.Īddress 0 is used for the broadcast address, which all slave devices recognize. Master addresses a slave by placing the slave address in the address field of the The individual slave devices are assigned addresses in the range of 1 – 247. Valid slave device addresses are in the range of 0 – 247 decimal. The address field of a message frame contains two characters (ASCII) or eightīits (RTU). Technically the Modbus Spec only allows slave addresses 1 to 247 (246 total slaves). For this purpose, Terminal 22 on the EtherMeter functions as a signal reference point with a built-in 100 ohm current-limiting resistor.Address 0 is reserved as a "broadcast" address where the master can write to the slaves, however the slaves are not supposed to respond to broadcast requests. Each reference tie should utilize a resistor to prevent large currents from traveling on the reference wire. Furthermore, the signal grounds of all devices should be tied together to a common voltage reference. 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. 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. It consists of two 22 gauge shielded/twisted pairs (red/black and green/white). The cost is generally around $3/ft.įor more economical 2-pair wiring, I recommend Belden 8723 cable. 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 22 gauge pair (Red/Black) is designated for 24VDC and GND. The 24 gauge pair (Blue/White) has a characteristic impedance of 120 ohms, which is optimal for RS485 transmission.
#MODBUS RTU WIRING PLUS#
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.įor 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. This ensures that signaling between all devices are referenced to a common ground potential. 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. Switch 4 – UP (LCD Backlight ON), or DOWN (Power-Saver Mode).Switch 3 – UP (Use 120 Ohm Terminator for RS485), DOWN (No Terminator).Switch 2 – DOWN (RS485 Mode), UP (RS232 Mode).Switch 1 – DOWN (Run Mode), UP (Setup Mode, Modbus/DF1 OFF).
In order to use RS485, attention should be given to the EtherMeter’s dip switch positions: 21 – Signal GND Reference (contains current-limiting resistor).When using the RS485 port, the following EtherMeter terminals are used: Jim Mimlitz, SCADAmetricsThe EtherMeter contains an RS485 port for Modbus/RTU and DF1 communications in multi-drop applications.