ABB REL 551-C1 2.3 Series Installation And Commissioning Manual page 58

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Configure the interface modules for V.36,
X.21 and RS530
Designation Explanation
REQ SEND Request to send (follows auxiliary voltage)
RXD Received data
SCREEN Connection of cable screen
TCLK DCE Transmitter signal timing from DCE
TCLK DTE Transmitter signal timing from DTE
TXD Transmitter data
For the co-directional operation the transmission rate of the transmitted signal must be
set.This setting, 56 or 64 kbit/s, is done on the HMI under:
Configuration/TerminalCom/RemTermCom/BitRate
For X.21 and contra-directional operation no settings are available.
For the signals used by the protection, the communication module for V.36 also fulfils
the older recommendation for V.35.
Chapter 8
Configuring the digital
communication modules