RM-01

Overview

The RM-01 devices are designed to automatically form a wireless mesh network upon being powered on. In this radio network one RM-01 device that is connected to a Modbus master (typically SCADA or PLC) is configured to work in master mode, while the other devices, connected to Modbus slaves are configured in slave mode. The job of the RM-01 devices configured in slave mode is to detect (using the Modbus slave detection mechanism described below) the addresses of each and every Modbus device connected to a particular RM-01. This information is sent to the RM-01 master device. In such network, each time the Modbus master sends out a command, the RM-01 that is working in master mode forwards this message to the proper RM-01 working in slave mode, to which the recipient of the Modbus packet is connected. If a Modbus packet is produced in response, the RM-01 slave sends it back to the RM-01 master, which in turn forwards it to the Modbus master device.

Mode of operation

The RM-01 can be configured to work in two modes:

• the master mode, in which it is connected to a single Modbus master device
• the slave mode, in which it is connected to one or more Modbus slave devices

In master mode, the RM-01 accepts Modbus requests from a Modbus master device (typically SCADA or PLC). Each time a valid Modbus packet is received, the RM-01 decodes the request and checks, if the recipient of the packet is known, meaning that the RM-01 master knows to which of the RM-01 slaves this Modbus packet should be forwarded. If the recipient is known, the packed is forwarded to this particular RM-01 slave. In cases when the recipient is not known, the RM-01 master broadcasts the request to all RM-01 slaves, expecting response from one of them. If the response is received, the RM-01 replies to the Modbus master device and saves the information about the origin of the response. This way, next time the request to this Modbus address is issued, the master RM-01 knows to which RM-01 slave it should be forwarded.

In slave mode, after being powered up the RM-01 first tries to detect the Modbus devices, connected to its RS-485 bus using Modbus slave detection algorithm (described below). Once it?s done, the information about the Modbus devices is sent to the master RM-01. If a Modbus packet is sent to the RM-01 slave, it is being transmitted on the RS-485 bus. If a valid response is received it is being forwarded back to the RM-01 master.

Modbus slave detection

When RM-01 is configured to operate in slave mode it automatically detects the Modbus devices connected to its RS-485 bus. It does that by issuing a READ COIL STATUS (FC=01) with coil (data) address = 0 and data count = 1 to each of the potential Modbus addresses. The scanned address range can be limited using the configuration software. The issued command is a valid Modbus command (with a valid CRC). If a Modbus device with a given address exists on the bus, it should respond with either the READ COIL STATUS RESPONSE or the INVALID REQUEST EXCEPTION. Either way, the RM-01 will then know that a Modbus device with particular address is present on its RS-485 bus. This information is stored in the slave RM-01 operating and also sent to the master RM-01.

The Modbus slave detection algorithm is triggered each time the slave RM-01 device is powered on. It is also triggered remotely by the master RM-01 in case, when the master RM-01 is restarted and it needs to update the information from the field. The scanning procedure has always the lowest priority on the RS-485 bus and does not interfere with the actual Modbus requests and responses.

The RM-01 is configured using a dedicated PC application. The connection is established by connecting the device to a PC using a mini-USB to USB-A standard cable. Consult the device datasheet for the detailed explanation of the settings.

Picture below presents a simple exemplary network consisting of a single Modbus master with an accompanying RM-01 media converter (operating in master mode) and five Modbus slave devices connected through three RM-01 media converters.

The picture also presents an exemplary configuration for each of the RM-01 devices. In order to communicate, each RM-01 must have the same wireless network name (and security password, if set). The RS-485 baud rates can be specified separately for each RM-01 operating in slave mode. Each RM-01 can also scan different range of Modbus addresses with various timeout settings.