Dnp3.0 And Modbus - Advanced Power technologies TTC-1000 Instruction & Operation Manual

6.2 DNP3.0 and MODBUS

Units equipped with the optional DNP3.0 or MODBUS communications interface
contain a plug-in Communications Processor module. The module contains a
separate microprocessor to handle all overhead functions associated with the
DNP3.0 or MODBUS protocols without affecting operation of the transformer
cooling control and monitoring. The module contains either a half duplex, full
duplex isolated RS-485, or multi-mode fiber optic asynchronous communications
interface capable of supporting multi-drop topologies. RS-485 interfaces differ
from RS-232 in that RS-485 uses a differential receiver and transmitter pair. This
permits RS-485 links to send and receive data over much greater distances as
long as some simple rules are followed. See Sections 3.7.3, 3.7.4, and 3.10 for
connections, jumper settings, and shield grounding.
After making the proper connections to the TTC-1000, there are only two settings
that need to be made: Node Address and Baud Rate. Node Addresses for
DNP3.0 can be any number from 0 to 65535. For MODBUS the addresses are
1-247, 0 is for broadcast and remaining 248-255 are reserved. Please consult
the DNP3.0 or MODBUS reference materials as some higher order addresses
are reserved for broadcast messages.
Baud Rates can be set to either 1200, 2400, 9600 or 19200. It is recommended
that both Node Address and Baud Rate settings be made before attempting to
communicate, however these settings can be changed "on-the-fly" without
powering down the TTC-1000. In the TTC-1000, the MODBUS RTU interface is
configured using 8 data bits, 2 stop bits, and no parity. The DNP3.0 interface is
configured using 8 data bits, 1 stop bit, and no parity.
The TTC-1000 implements DNP3.0 Level 1 communications. This includes Class
0 polls (Object 60 Variation 1) of analog and binary output points. The TTC-1000
supports Object 1 Variation 2 binary outputs. Binary outputs include all
temperature and load set points along with the state of each output relay.
Because Variation 2 is supported, the TTC-1000 communicates whether or not
an output relay is under manual control by exerting the "forced" status bit for
these points only. The "forced" status bit is located in bit 4 of each binary output
octet transmitted to the Master. For Analog Outputs, the TTC-1000 supports
Object 30 Variation 4. These are 16 bit signed analog quantities without status.
Included in the Analog Output points are all temperatures equipped plus load
data if available. Also, the TTC-1000 uses two points to identify whether or not
the probe is measuring Top Oil, Heated Well, LTC Differential or Ambient
temperature. This is particularly useful for dual probe units. It is noted that Object
1 Variation 2 and Object 30 Variation 4 points cannot be read individually and
can only be read by a Class 0 poll.
In addition, the TTC-1000 supports the remote control of the four output relays.
The TTC-1000 supports both direct control (Object 10 Variation 0) and Select
Before Operate control (Object 12 Variation 1). The TTC-1000 permits the user
to command an output relay to pickup. It cannot be used to dropout an output
relay. If the Binary Input point is turned off, it merely returns the device to local
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