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392
Serial communication, SPA
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The SPA communication is mainly used for SMS. It can include different numerical re-
lays/terminals with remote communication possibilities. Connection to a personal com-
puter (PC) can be made directly (if the PC is located in the substation) or by telephone
modem through a telephone network with CCITT characteristics.
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REx 5xx
REx 5xx
(Minute pulse
from station clock)
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The SPA protocol V2.5 is an ASCII-based protocol for serial communication. The com-
munication is based on a master-slave principle, where the terminal is a slave and the
PC is the master. Only one master can be applied on each optic fibre loop. A program
is needed in the master computer for interpretation of the SPA-bus codes and for trans-
lation of the settings sent to the terminal.
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When communicating locally with a Personal Computer (PC) in the station, using the
rear SPA port, the only hardware needed for a station monitoring system is:
•
Optical fibres
•
Opto/electrical converter for the PC
•
PC
SPA bus
Optical fibre
loop
Telephone
modem
Opto/electrical
converter
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CAP tool
Telephone
modem
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