When a query is received, it will either perform an action or reply with a response to the master. A slave device spends its time monitoring the network for queries specifically addressed to it. In a Modbus RS485 Network, communication begins when the master device sends a query to a connected slave. The master listens for the response and if not obtained in an appropriate time-frame will terminate the communication. The master performs the role of the transmitter, making requests to a designated slave which acts as the receiver. The transmitter then waits a pre-determined length of time for a reply or decides that no reply will be forthcoming from the receiver.Īn RS485 network is built based on a master/slave framework. Communication is enacted by a transmitter making a request directed at a specific receiver. If you desire more information regarding RS485 you can consult the EIA (Electronic Industries Association) or companies such as Texas Instruments or Maxim Semiconductors that manufacture devices compatible with the protocol.Ī half-duplex system is comprised of one or more transmitters and receivers where only one transmitter can be active at a time.

Minimum receiver input Resistance (kohms) Parameterĭriver Output short circuit current Limit (mA) RS485 provides half-duplex transmission on a balanced line covering distances of up to 1.2 km. That is the purpose of this brief Modbus tutorial.īelow is a table that summarizes the characteristics of the RS485 protocol. When moving from working with the ASCII protocols of RS232 devices, it is advisable to learn the differences involved with the Modbus protocol. This less expensive approach removes the requirement for any additional components to be used.ĭevices using RS485 ports usually employ the Modbus protocol. You can construct a gateway between RS232 and RS485 interfaces, but usually you will see the RS485 protocol connecting directly to USB, GSM or Ethernet ports. You can do this by using a bus terminator, moving a switch or with a small resistor screwed onto a terminal.Ĭare must be taken to ensure that the correct interface is being used as the RS485 and RS32 protocols are not completely compatible. This eliminates the need to have several interfaces available when querying multiple devices.

One of the reasons that RS485 interfacing is employed in industrial settings is its ability to serve several devices attached to the same bus. The two protocols use different electric signals to enable data transmission. RS485 is a protocol similar to RS232 that is used to implement serial data communication.

Ccs c modbus rtu