ABB DSPA 110 Automatic card

Redundant systems can be implemented using multiple CIs. Redundant links can also be configured from a single CI, though this will result in the module being a single point of failure. Facilities are provided to the application programmer to actively control the use of the communications links, via application network variables defined by the user in the System.INI file, as described in section 3.5. If a Slave device fails to respond to three consecutive messages then a communications failure for that Slave will be logged (the number of messages may be configured in ‘Retries’). A communications error to a Slave device on a particular link results in the suspension of polling of that Slave for a period of time defined by the ping ‘Repeat Rate’. This allows the polling of other devices on the same Master to still be polled without pausing for the communications time-out on each cycle through the poll.

Modbus Master Polling Sequence

 Once a Modbus Master begins polling, one message from each Slave connected to the Master is executed in turn. This is repeated for all the messages listed for each Slave. Once all the messages for a given Slave have been executed, then the list will be repeated from the first message again. Figure 16 below shows the sequence of three Slaves connected to a single Master being polled.The resultant message sequence on the communications link will be as shown in Table 14 below.

Message Sequence 

Where a Slave has a minimum packet gap configured, each subsequent message for that Slave will be checked as in the sequence above. However, no message will be sent until the minimum separation time between consecutive messages has been exceeded. If a message in the Slave sequence is disabled using an optional control variable, then at the appropriate poll time the message will be skipped and the next enabled message for that Slave will be sent instead. If all the messages for a Slave are disabled, then the Slave will be pinged in the same manner as if the Slave had been commanded to standby mode.

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