Schweitzer Engineering SEL-2032, Instruction Manual

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9.16
SEL-2032 Communications Processor Instruction Manual Date Code 20120126
Protocols
Modbus RTU Protocol
A successful SEL-2032 response will have the format:
1 byte.
Slave Address
1 byte.
Function code (03h)
1 byte.
Byte count (should be twice number of registers read)
n bytes.
Byte Count Bytes of Data
2 bytes.
CRC–16 for Message
Read Input Register (Function Code 04h)
The SEL-2032 uses function code 04h to read specific data from a Modbus
map. This map contains various types of data at discrete addresses depending
on relay type, but independent of settings. You must first collect relay meter
data with the
20METER command in order for it to be visible in the register
map. You can then use the MAP_TYPE setting to select whether the
SEL-2032 uses the default map, which includes floating-point data, or an
integer-only map. You may read a maximum of 125 registers at once with
function code 04h.
Function code 04h provides a method for a Modbus master to read various
relay data values directly and receive scaled values without the requirement of
math/movement settings. Conversions are done internally based on the type of
data being read. Essentially, the Modbus master can do direct reads of
magnitudes, angles, timestamps, etc., and get automatically scaled values that
retain most of the resolution of the original floating point numbers.
The master request must have the following format:
1 byte.
Slave Address
1 byte.
Function Code (04h)
2 bytes.
Starting database address
2 bytes.
Number of registers to read
2 bytes.
CRC–16 for message
A successful SEL-2032 response will have the format:
1 byte.
Slave Address
1 byte.
Function Code (04h)
1 byte.
Byte Count (should be twice number of registers read)
n bytes.
Byte count bytes of data
2 bytes.
CRC–16 for message
Table 9.8 to Table 9.14 show examples for integer type data from various SEL
relays for 04h accesses. You can use the MODMAP command to identify the
registers that contain the data you want to collect through the Modbus port.
Most masters use 3X references when accessing input registers. To use this
reference method with 5-digit addressing, simply add 30001 to the address in
the
MODMAP command response. The User Region begins at address
32401. Ensure your master can accept floating-point data transferred with the
most significant word first before configuring the SEL-2032 to use them.
While accessing relay data using function code 04h does not require
processor-intensive math/movement settings, you should consider overall
system performance before using this data access method. For example, if you
connect 10 SEL relays to your SEL-2032 and a Modbus master on Port 14,
you could use function code 04h to read data from the relay register maps.
Courtesy of NationalSwitchgear.com