en01000134.vsd
Start
flag
Information CRC
Stop
flag
8 bits n x 16 bits 8 bits16 bits
IEC01000134 V1 EN
Figure 501: Data message structure
The start and stop flags are the 0111 1110 sequence (7E hexadecimal), defined in the
HDLC standard. The CRC is designed according to the standard CRC16 definition.
The optional address field in the HDLC frame is not used instead a separate addressing
is included in the data field.
The address field is used for checking that the received message originates from the
correct equipment. There is always a risk that multiplexers occasionally mix the
messages up. Each terminal in the system is given a number. The terminal is then
programmed to accept messages from a specific terminal number. If the CRC function
detects a faulty message, the message is thrown away and not used in the evaluation.
When the communication is used for line differential purpose, the transmitted data
consists of three currents, clock information, trip-, block- and alarm-signals and eight
binary signals which can be used for any purpose. The three currents are represented as
sampled values.
When the communication is used exclusively for binary signals, the full data capacity
of the communication channel is used for the binary signal purpose which gives the
capacity of 192 signals.
Section 18 1MRK505222-UUS C
Remote communication
1070
Technical reference manual
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