Page 383 - Gas Detection Systems Training Manual

Note: SCAN MODE: A crucial feature of the SEC 3120 16-bit Modbus implementation is that it enables a Modbus Master to update one or more holding register contents at a time. This allows the master to start at any legal holding register address and update in one scan block, an entire set of relevant parameters for a given sensor, such as all Alarm Set Points in one write operation.
This is especially useful for updating a sequence of 16-bit registers that need to be written together to be valid and protect against unwanted or accidental write operations. In the case of the IEEE 754 32-bit Floating Point Alarm Set Points, a triad of registers is required to fully change a given set point, all in one action otherwise the operation will be rejected as a password failure. Since a 32-bit floating point value requires two 16-bit registers to hold the entire contents, a set of three registers is designated;
1. Password enable register- must contain the Hexadecimal value 4259. 2. High 16-bit word of IEEE 754 encoded floating point set point value. 3. Low 16-bit word of IEEE 754 encoded floating point set point value.
A state machine ensures that the sequence order is followed and that the password enable register occurs first in the multi-write operation and contains the correct value. If the sequence is not followed correctly or the password is incorrect, the set point will not be updated and an exception generated. This prevents un-intended write operations.
At the same time, it allows multiple-word values or blocks that make up a huge value to be written, without breaking the Modbus 16-bit specification or creating a burdensome long set of 16-bit operations.
In both multiple read and write operations, it allows a low-level Master driver to determine whether it uses a single or multiple register operation, thus enabling better efficiency in the Modbus Master.
SEC 3120 16-bit Modbus Manual Page 43 Of 46 Sensor Electronics Corporation For Version 1.2 and above December 2012 P/N 1580282, Rev. 1, 20121228