Serial Wiring and Speed

Modbus can be used over ethernet as well as serial cable. Here in this post we are going to see serial wiring and different wiring standards  for serial communication. One of the Modbus protocols is Modbus RTU. Modbus RTU devices use  one of these three electrical interfaces .

RS232

RS485

Rs422

RS232 : it is a simple point to point arrangement if you only need to connect one device to another and the distance between them is less than 50 feet or 15 meter.

 The original Modbus was RS-232-based, but the need to connect multiple slaves soon expanded Modbus to use multidrop wiring methods. RS-422 has been used but RS-485 is more capable and is more commonly used. To connect more than two devices on the same line and have a distance of greater than 50 feet you should use RS485 or RS422.

RS485 : it is the most popular method of serial communication  this standard supports upto 32 nodes over a range upto  4000 feet  without a repeater .

Speed :  the speed at modbus messages are sent is referred to as baud rate (bits/second). All devices on an RTU network must use the same baud rate. Different devices support different transmission speed but the typical rage is 9600- 19200 b/s. Generally, the higher transmission speed, the shorter the cable and the more important to have the correct network end termination resistors to minimize reflections Modbus serial network has a master device that issues commands to slave devices . the slave will not transmit information unless they receive command to do so from the master there can be one master and a maximum upto 247 slaves each with unique slave id. A master writes data to the slave as well as reads data from the slave.