Is Modbus the same as RS485 - RS485 communication tutorial
RS485 is a serial data transmission standard widely used in industrial implementations. The Modbus protocol is commonly used when implementing RS485 communication. This differentiates RS485 from the RS232 protocol which communicates by transmitting with ASCII characters. It is important to understand Modbus if you will be working with devices using the RS485 protocol.
If you are a technician responsible for supporting RS485 devices, there are some techniques that can make your job easier. We present this short tutorial that demonstrates the differences in the Modbus vs RS485 protocols.
- What is RS485 communication?
- What is Modbus?
RS485 is a protocol similar to RS232 that is used to implement serial data communication. The two protocols use different electric signals to enable data transmission.
One of the reasons that RS485 interfacing is employed in industrial settings is its ability to serve several devices attached to the same bus. This eliminates the need to have several interfaces available when querying multiple devices. You can do this by using a bus terminator, moving a switch or with a small resistor screwed onto a terminal.
Care must be taken to ensure that the correct interface is being used as the RS485 and RS32 protocols are not completely compatible. 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. This less expensive approach removes the requirement for any additional components to be used.
Devices using RS485 ports usually employ the Modbus protocol. When moving from working with the ASCII protocols of RS232 devices, it is advisable to learn the differences involved with the Modbus protocol. That is the purpose of this brief Modbus tutorial.
Below is a table that summarizes the characteristics of the RS485 protocol. RS485 provides half-duplex transmission on a balanced line covering distances of up to 1.2 km.
|Mode of Operation||Differential|
|Number of Drivers and Receivers||32 Drivers |
|Maximum cable length (metres)||1200|
|Maximum data rate (baud)||10 M|
|Maximum common mode voltage (Volts)||12 to -7|
|Minimum Driver Output Levels (Loaded)||+/- 1.5|
|Minimum Driver Output Levels (Unloaded)||+/- 6|
|Drive Load (Ohms)||60 (min)|
|Driver Output short circuit current Limit (mA)||150 to Gnd, 250 to -7 or 12 V|
|Minimum receiver input Resistance (kohms)||12|
|Receiver sensitivity||+/- 200mv|
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.