Sensor Electronics SEC 3000, Instruction And Operation Manual

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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
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.