Modbus Specifications

Connection Port

Each device [MASTER or SLAVE] has a connection port with 2 terminals that are used to communicate with that particular device over Modbus RS485.  These terminals are mostly marked as A or + and B or -.  The wiring is done such that all these devices are connected in parallel.  This means that all the A/+ terminals should be connected together and all the B/- terminals should be connected together.

Inverting the A/+ and B/- connections of a device does not only prevent it from communicating but may also stop the entire communication system from working, owing to incorrect direct (polarisation) voltage found on the terminals of the incorrectly connected device.  Due to this, it is advised to use the same colour cable for all A/+ terminals, and a different coloured cable for all B/- terminals to avoid incorrect wiring.

Connection Between Devices

The manner in which the devices are connected in parallel is important. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible.

Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data.

The easiest and the best way to connect the devices in a Modbus RS485 communication is using the daisy chain method.  It is shown below.  Any other way of connecting the devices together is not recommended as it might cause communication issues or damage to the overall system.

Terminating Resistors

Transmission line effects often present a problem in data communication networks. These problems include reflections and signal attenuation.  When a voltage is first applied to an RS485 line, current flows through the line (even if the line wires are open).  Shortly after reaching the end of the line, the current settles to a final value determined by the series resistances in line, the voltage applied, and termination.  The line sees reflected voltages as the current settles if the initial and final currents vary, resulting in possible data integrity issues.

In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable.  The end resistance must be used only at the ends of the main cable.  If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable.  If the data rate is low or cables are short, termination may be unnecessary.  As data rates and/or cable lengths increase, which is most cases, termination becomes mandatory.

To eliminate the presence of reflections from the end of the cable, it must be terminated at both ends with a resistor across the line (between A/+ and B/-), in accordance with its characteristic impedance.  Both ends must be terminated since the direction of propagation is bidirectional. In case of an RS485 twisted pair cable this termination is typically between 120 and 130 Ω.  

Grounding/Earth Connection.

The cable shield must be earthed only in one point. Normally, this connection is made at one end of the main cable.  Properly grounding Modbus is important, especially when working close to high voltage sources.  An instrument that is not appropriately grounded can give false readings, fail completely or turn  into a safety hazard.  If you are encountering noise or irregular or abnormal problems with a Modbus serial link, the problem is likely related to grounding, incorrect shielding, or wiring power wires next to Modbus wiring.

Recommended wiring connection of RS485 is using a single shielded twisted pair (STP) cable. Using a twisted pair cable cancels out interference and ensures that the 2 conductors are at the same lengths. Having a ground shield on the twisted pair cable further provides protection to cross-talk and interference.

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