RC Systems ST-72, Instruction Manual

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ST-72 User's Manual
Document: UM-1007 Revision Level A
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SECTION 5
MODBUS
In compliance with CSA C22.2 No. 152 certification, the Wireless /
MODBUS interface to a LEL gas detector may only be used for data collection
or record keeping with regard to combustible gas detection and not for
performance verification.
The ST-72 is equipped with two standard RS-485 ports that can be independently
configured as Modbus
master or slave, an optional RS-485 Isolated piggyback Board
(part # 10-0368) may be added to the I/O Board via ribbon cable J5. This option adds
two additional isolated RS-485 ports for a total of four. These two additional serial ports
can also be configured for either Master or Slave. Section 5.0 defines register locations
of data available via the ST-72 slave port.
5.1 MODBUS TCP
In addition to the RS-485 ports, the ST-72 supports both master and slave ModbusTCP.
ModbusTCP is always enabled through the Ethernet port. See section 2.5 for Modbus
configuration options. The ModbusTCP slave is always active on port 502. The unit can
be polled by its IP Address or hostname. When ModbusTCP slave is used, the slave ID
field of the message is ignored. Channels can be configured to poll using the ST-72’s
ModbusTCP master interface. Devices are polled by IP Address, not hostname.
5.2 MODBUS SLAVE WRITES
The Modbus slave ports allow function code 5 (write coil), as well as function code 6,
and 16 (write holding registers). These function codes can be used to write
configuration parameters to the ST-72. By default, all Modbus writes are disabled
except the unlock parameter 40099. The Modbus lock code can be written to register
40099 to enable writes to other registers. The unit will be unlocked for 10 minutes after
the last write occurs. After the 10 minute timeout, the unit will automatically save any
parameters that have been written. All written parameters can be saved manually by
writing a value of 1 to coil 95 or register 40095.
Writing parameters that span multiple registers (such as 32bit floating points) requires
function code 16. All of the registers must be written at once.
5.3 MODBUS SLAVE REGISTER LOCATION
The following tables describe the ST-72’s Modbus slave database. Any portion of this
data may be read by a Modbus master device such as a PC, PLC or DCS. Since the
Modbus port is RS-485, many ST-72s may be multi-dropped onto the same cable.
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