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Remote end data communication
Optical connection of multiplexer is only possible if the multiplexer is of type
FOX6Plus or FOX20 from ABB. The terminal can then be connected optically to the
multiplexer, provided the protection is equipped with the optical fibre modem , not the
short range fibre optical modem, and the FOX is equipped with an Optical Terminal
Module of type N3BT.
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The Remote end data communication consists of two parts, one software part that han-
dles the message structure, packing different pieces together, activate sending of the
messages, unpacking received messages etc, and one hardware part forming the inter-
face against external communication equipments. The hardware part, or built-in mo-
dems, can have either galvanic or optical connection. To ensure compatibility with a
wide range of communication equipment and media, the terminal is designed to work
within the signalling bandwidth of a standard CCITT PCM channel at 64 kbits/s. To en-
able the use in North American EIA PCM systems working at 56 kbits/s, some of the
interfacing modules can be adapted to this bit rate.
The message is based on the HDLC protocol. This is a protocol for the flow manage-
ment of the information on a data communication link that is widely used. The basic
information unit on an HDLC link is a frame. A frame consists of:
•
start (or opening) flag
•
address and control fields (if included)
•
data to be transmitted
•
CRC word
•
end (or closing) flag.
The start and stop flags are 8 bit each and the Cyclic Redundancy Check (CRC) 16 bits.
The data field differs if between a message sent from a slave to a master and a message
sent from a master to a slave. The principle design is according to figure 200 "Data mes-
sage structure" on page 377.
Start
Information
flag
8 bits
n x 16 bits
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Stop
CRC
flag
16 bits
8 bits
en01000134.vsd
&KDSWHU
'DWD FRPPXQLFDWLRQ
377
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