Accessing Data Via The User Map - GE Multilin 745 Communications Manual

CHAPTER 2: MODBUS PROTOCOL
Accessing data via the
user map
745 TRANSFORMER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
Only the data for a single channel of a single trace memory buffer can be read from the
Modbus memory map at a time. The "Trace buffer selector index" register at address
4005h selects the trace memory buffer, and the "Trace channel selector index" register at
4006h selects the trace memory channel, whose waveform data can be read from the
memory map. For example, to read the waveform data for the "Winding 1 phase C current"
of trace memory buffer 5, the value 5 must be written to the trace buffer selector index,
and the value 2 (as per data format F65) must be written to the trace channel selector
index. All the captured waveform data for buffer 5, channel "Winding 1 phase C current"
can now be read from the trace memory data registers at addresses 4010h to 4416h. Only
the trace memory buffers for the last three trace memory triggers are actually stored in
the relay's memory. Attempting to retrieve data for older triggers that are not stored will
result in a Modbus exception response when writing to the trace buffer selector index.
For example, to retrieve information from the trace memory, a SCADA system polls the
total number of trace triggers register once every minute. It now reads a value of 6 from
the register when previously the value was 5, indicating that one new trigger has occurred
during the last minute. The SCADA system writes a value of 6 to the trace buffer selector
index register. It then writes the value of 0 to the trace channel selector index register,
reads the waveform data for "Winding 1 phase A current" of trace buffer 6 from the trace
memory data registers, and stores the data to memory for retrieval by an operator. The
SCADA system now writes the value 1 to the trace channel selector index and then reads
the waveform data for "Winding 1 phase B current". The SCADA system continues by
writing all other channel numbers to the trace channel selector index, each time reading
the waveform data, until all channels for buffer 6 have been read. All the waveform data for
the new trace memory trigger has now been retrieved by the SCADA system, so it resumes
polling the total number of trace triggers register.
The 745 has a powerful user map feature that allows a computer to access up to 120 non-
consecutive registers (setpoints or actual values) by using one Modbus read message.
It is often necessary for a master to continuously poll various values in each of the
connected slave relays. If these values are scattered throughout the memory map,
reading them would require numerous transmissions and would labor the communication
link. The user map can be programmed to join any memory map address to one in the
block of consecutive user map locations, so that they can be accessed by reading (and
writing to, if joined to setpoints) these consecutive locations. The user map feature consists
of:
1. User map addresses 1 to 120 (located at memory map addresses 0180h to 01F7h).
These are the setpoints which store the (possibly discontinuous) memory map
addresses of the values that are to be accessed.
2. User map values 1 to 120 (located at memory map addresses 0100h to 0177h). These
are the access points of the remapped locations. Reading user map value 1 returns
the value at the address stored in user map address 1, user map value 2 the value at
user map address 2, and so on. Writing to any user map value is only possible if the
address stored in the corresponding user map address is a setpoint value.
MODBUS PROTOCOL
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