Emerson S-series Serial Interface Redundancy Made Easy

Redundancy Made Easy

The Redundant Serial Interface consists of a pair of serial

cards and a redundant terminal block. The cards each

contain two serial communications ports that support

RS422/485 half duplex (in either single slave or multi-drop

configurations), or RS422/485 full duplex (in single slave

configuration only), as well as RS232. These ports are

individually configurable and support data rates up to

115 KBaud.

Slave devices must provide two serial ports in order to allow

the active and standby serial cards to be connected. The

Active card continually poll input data and writes output

data to the device on one serial link, while the standby card

periodically polls the slave device for a diagnostic register.

Verify with the device supplier for its ability to support

redundant Modbus serial communications.

When a fault is detected, the system automatically

switches to the backup serial card. Events that can cause a

switchover include:

• Hardware failure within the active card

• Loss of Communications (including field cable and

slave port problems) between the active card and the

slave device

• Removal of the active card from the carrier

A switchover may also be initiated from control logic. This

allows additional watchdog logic to monitor multiple slave

devices for communication faults not detected by the

Serial Interface. A manual switchover can be initiated from

diagnostics explorer.