Date Code 20010518 Appendix G G-1
SEL-2020 Instruction Manual
APPENDIX G: MODBUS PROTOCOL
OVERVIEW
The SEL-2020 Communications Processor supports the Modbus RTU protocol on ports 12, 14,
and 16 for data access from any SEL-2020 port. The SEL-2020 is always a Modbus slave. All
data within the SEL-2020 database can be read using Modbus. Basic control functions can be
performed using Modbus.
You can set the SEL-2020 for up to 16 Modbus slave addresses; set a unique slave address for
each port that has data you want to collect through a Modbus port. The Modbus master views
the SEL-2020 as a group of individual devices, each with a unique Modbus slave address.
Modbus protocol compatibility facilitates connection to many Remote Terminal Units (RTU),
and to most Programmable Logic Controllers (PLC) and PLC Networks.
SETTINGS
From Port F or another master port, use the SET P 12, SET P 14, or SET P 16 command to set
the device type to “MASTER” and the protocol to “MODBUS”. The SEL-2020 will prompt for
the map style, device ID offset, and Modbus slave address for each of 16 ports. With the map
type setting, you select between the default map which contains floating-point data and the
integer-only map. With the device ID offset setting, you can select an offset for the device ID
table, which is discussed later in this appendix. You must also provide the slave device
address(es) for Modbus access to data from the desired port(s).
For each SEL-2020 port connected to an IED with data you want to access by Modbus, you must
use the following SET commands (refer to Section 6: Settings):
• Use the SET P command to set and autoconfigure 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. You can also access user region data using this
function code.
• For function code 03 access, set the collections as desired and use the MAP command to
determine the data addresses.
• For custom data access, use SET M to organize and scale data as desired.
HARDWARE CONNECTIONS AND RTS LINE USAGE
An EIA-232 Connection is the most common connection between an SEL-2020 Communications
Processor and a Remote Terminal Unit. When Modbus is used in a dedicated link, the RTU
should ignore the RTS output from the SEL-2020. To accomplish this, you may need to connect
the Clear-To-Send (CTS) pin to +12 Vdc in the cable connector at the RTU.
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