Sensor Electronics SEC 3000, Instruction And Operation Manual

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SEC 3120 16-bit Modbus Manual Page 33 Of 46 Sensor Electronics Corporation
For Version 1.2 and above December 2012 P/N 1580282, Rev. 1, 20121228
SEC 3120 units support 40+ holding registers for each sensor attached, as well as
an additional 12+ holding registers specific to the 3120 unit head itself as listed in Table
7.
If a starting address is not within the range shown in Table 7, the SEC 3120 unit
will issue an Exception 2 as a response. If the number of points is too many, the SEC
3120 may issue an Exception 3 as a response.
Note: SCAN MODE:
A crucial feature of the SEC 3120 16-bit Modbus implementation
is that it
enables a Modbus Master to gain one or more holding register contents at a
time.
This allows the master to start at any legal holding register address and retrieve in
one scan block,
an
entire set
of relevant parameters for a given sensor, such as Gas
Concentration, Operating Status and Alarm Set Points or more in
one read query
operation.
Function Code 06 - Preset Single Holding Register
Description
Presets a value into a single holding register (4x references). Modbus allows a
broadcast mode with function presets to the same register reference in all attached slaves,
however the SEC 3120 supports multiple sensors attached to a single unit, hence
broadcast mode is not allowed, except for one preset register pair 42920 (listen only
mode) and 42921 (restore from listen only mode).
To accomplish the same functionality as a broadcast to the same preset reference
in the SEC 3120, so long as the sensors attached are truly identical, region 0 of the
holding register map (holding register address range 42001 - 42099) may be used. If
written to the same relative offsets as the sensor regions (i.e. 42004 for 42x04, 42005 for
42x05, where x = specific sensor number) then the values written to region 0 registers
will apply as if written to all sensor relative region offset registers.
The advantage of using a preset function is that it does not have the overhead of
an item count or a byte count. It is a very small packet targeted at changing one very
specific 16-bit register with a full read-back of the contents to ensure proper
communication.
Response Exception 2
Field Name Example (Hex)
Slave Address 01
Function Code 83
Exception Code 02
CRC Low Byte C0
CRC High Byte F1
Response Exception 3
Field Name Example (Hex)
Slave Address 01
Function Code 83
Exception Code 03
CRC Low Byte 01
CRC High Byte 31