Motorola 68230 - Vanguard 300 DSU/CSU Operator's Manual page 70

Differences in CTP Reporting of Output Signals
Use the CTP "Monitor" function from the Main menu to view these signals.
Input Signal
RS (Reset)
LL (Local Loopback)
CL (CSU Loopback)
IDL (Idle)
CLK (Clock Mode)
Differences in CTP Statistics
CTP statistics output also differs in the following ways:
Detailed Port Statistics show DSU input/output signaling, as well as
noting the installed DSU.
Detailed Node Statistics show DSU installation.
CTP Loopback Modes
Two additional CTP loopback modes now exist.
Mode
DSU Internal
Loopback
DSU Internal and
External Loopback
Function
Resets the DSU option hardware. Normal operating state
is "H."
When "L", the DSU hardware is locally looping data
back to the Vanguard hardware.
When "L", the DSU hardware is looping the remote
connection's receive to transmit.
An "H" informs the DSU hardware to send idle bipolar
violations to the remote system.
When "H", denotes network clock use. When "L", the
DSU option provides the clock.
Function
Does not affect the external interface. Loops the local
transmit data back to the receive data. Failure indicates
no connection.
Performs the internal loopback by looping the external
transmit to the external receive to allow manual testing of
the remote interface.