9.11
Date Code 20120126 Instruction Manual SEL-2032 Communications Processor
Protocols
Modbus RTU Protocol
device ID table, which is discussed later in this section. You must also provide
the slave device address(es) for Modbus access to data from the desired
port(s).
For each SEL-2032 port connected to an IED with data you want to access by
Modbus, you must use the following SET commands (refer to Section 7:
Settings):
➤ Use the SET P command to set and auto-configure the SEL
IED port.
➤ For function code 04 access, use the SET A command to
specify 20METER, 20DEMAND, 20HISTORY, 20TARGET,
and 20BREAKER data retrieval as desired. The region selected
for the data collection does not matter; Modbus will access the
first data region of that type on the port. Scaling and
conversions are handled automatically with function code 04.
You can also access user region data using this function code.
Because of the amount of internal processing performed on
these reads, system performance may be affected (see Read
Input Register (Function Code 04h) on page 9.16).
➤ For function code 03 access, set the collections as desired and
use the MAP command to determine the data addresses.
Function code 03 direct database reads may require scaling or
conversion by using SET M commands. See below and Read
Holding Register (Function Code 03h) on page 9.15 for more
information.
➤ For custom data access, use SET M to organize and scale data
as desired. This is the most efficient method and provides the
best update performance to the Modbus master.
Hardware
Connections and
RTS Line Usage
An EIA-232 connection is the most common connection between an
SEL-2032 and an RTU. When Modbus is used in a dedicated link, the RTU
should ignore the Request-To-Send (RTS) output from the SEL-2032. To
accomplish this, you may need to connect the Clear-To-Send (CTS) pin to
+12 Vdc in the cable connector at the RTU.
If you use the SEL-2032 as a slave in a multidrop Modbus configuration, use
the RTS output as your push-to-talk signal to key on the slave transmitter.
Devices that typically utilize RTS keying include EIA-232 to 4-wire EIA-485
converters and modems bridged to a shared audio line. The SEL-2032 asserts
the RTS line prior to transmitting, executes the delay established by the
SETTLE1 setting, transmits a message, executes the delay established by the
SETTLE2 setting, and deasserts the RTS line. While the SETTLE2 (post
transmit) delay is executing, the RTS line remains asserted. So, a transmission
that occurs during the SETTLE2 delay will be sent without executing the
SETTLE1 (pretransmit) delay.
Data Access
Considerations
When you program the Modbus master device to read data, you may access all
data in one read message, or you may access selected data with separate read
messages. If you read all data, the data you read will all correspond to a single
data sample from the attached relay. However, if you read the data in pieces,
subsequent reads will not necessarily be from the same data sample. You can
avoid this by using the 05 function code to freeze a copy of the data for
reading.
Courtesy of NationalSwitchgear.com
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