Schweitzer Engineering SEL-2032, Instruction Manual

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9.12
SEL-2032 Communications Processor Instruction Manual Date Code 20120126
Protocols
Modbus RTU Protocol
Data can be stored in either an Archive data region or a normal database
region. If the data are stored in a normal database region, the data retrieved
through Modbus protocol are the newest collected data from the relays. If the
data are stored in an Archive data region, the data retrieved through Modbus
protocol are the oldest collected data from the relays.
To clear the oldest collected data in an Archive data region, send the clear
message (using function code 05h). After the oldest record is cleared, the next
record can be read. The clear only works for data in an Archive data region. If
the data are not stored in an Archive data region, the SEL-2032 will respond to
the clear with an exception message containing error code “Illegal Data
Address (02h).”
Floating-point data requires 32-bits or two 16-bit Modbus registers. The
SEL-2032 presents floating-point data as two consecutive floating-point
registers. For example, if you move the current from Port 3 to the Port 16 User
region with the
SET M line 000h,F = 3:METER:IA then you would read the
data from register F800h and F801h. The register addresses converted to
Modbus references are 46389 and 46390. The SEL-2032 stores floating-point
data in SEL standard format of MSW (Most Significant Word) in the first
register and LSW (Least Significant Word) in the second register. Modicon
PLCs store LSW in the first register and the MSW in the second register, so
Modbus drivers configured to work with floating-point data from Modicon
PLCs are not compatible with floating-point data from the SEL-2032.
Timing
The SEL-2032 should generally respond within 1 second of receiving a
Modbus request. In the rare case that database access is unavailable, the
SEL-2032 will respond with a Modbus busy signal. For Modbus function 03h
read requests, the SEL-2032 should respond within 1 second of receiving a
request, and typically within 0.5 seconds. Depending on processor and
communications loads, you may experience delays in response times for
Modbus function 04h read requests. See
Read Holding Register (Function
Code 03h) on page 9.15 and Read Input Register (Function Code 04h) on
page 9.16 for more information.
The Modbus protocol requires a 3.5-character silence to denote the beginning
or end of a message. The SEL-2032 monitors the elapsed time between receipt
of characters. If 3.5-character times elapse without a new character, then the
SEL-2032 ends the message and starts listening for a new transmission. All
messages received by the SEL-2032 must be separated by at least 3.5-
character times plus 2.0 ms to ensure there is no confusion between messages
(3.5-character times is 4.0 ms at 9600 baud).
Function Codes
Message Framing
All Modbus data requests consist of an address, a function code, some data,
and a checksum. For the SEL-2032 to respond, the address must match one of
those established in the settings, and the checksum must be valid. This frame
format can be viewed as follows:
1 byte.
Slave Address (must match an ADDRESS n setting)
1 byte.
Function Code (see below for supported function codes)
n bytes.
Information specific to function code
2 bytes.
CRC–16 code for message
Courtesy of NationalSwitchgear.com