Modbus Serial Port Settings

The Com’X is the master of the Modbus serial line. All the other devices connected to this serial line must be configured as Modbus slaves.

The slave device settings such as Baud rate, Parity, and Number of stop bits must match the Com’X. These settings are set by using the front display of each device.

This table describes the Modbus serial port settings:

Field	Description	Options
Baud rate	Defines the speed of the serial line.	1200, 4800, 9600, 19200, 38400, 57600, and 115200 Bauds.⁽¹⁾ The factory setting is 19200 to match the values of Schneider Electric equipment. Most third-party Modbus equipment supports this Baud rate.
Parity	Defines the parity bit of the transmitted bytes.	odd⁽²⁾ even none The factory setting is even to match the values of Schneider Electric equipment. Most third-party Modbus equipment supports this parity setting.
Number of stop bits	Defines the number of stop bits transmitted between 2 bytes.	1 or 2 The factory setting is 1 to match the values of Schneider Electric equipment. Most third-party Modbus equipment supports this number of stop bits.
Timeout	Defines the time the Com’X has to wait before generating an error for an unanswered Modbus request.	100 to 10000 ms The factory setting is 1000 ms to match the values of Schneider Electric equipment.
Terminator resistance	Has a value of 120 Ω.	Yes or No Terminator resistance must be activated if the Com’X is located at the end of the Modbus bus. Not following this rule can result in communication interruption.
Serial line polarization	Has a value of 510 Ω.	Yes or No It is recommended that the master polarizes the line. No other slave device has to support line polarization resistances or to activate serial line polarization resistances.
⁽¹⁾ A high Baud rate value reduces response time but may be more sensitive to disturbance. In case of disturbance, check the impedance on the serial line before reducing the Baud rate. ⁽²⁾ Even or odd settings enable a byte integrity check that may detect a transmission error at byte level. At this level, there is no advantage to use this check: Modbus protocol provides a CRC check that keeps the integrity of the whole Modbus frame.