IBM 3745 Maintenance Information Reference page 384

The MUX error register is reset by the hardware when it is passed to the
inbound serial link.
LIC Error Detection and Reporting
LIC Driver Check (Echo Check)
The driver check (echo check) is detected at LlC level, but the error reporting is
controlled in the FES by the RAM C masks (refer to "Process of LlC Errors" on
page 4-177 for more information).
Modem-Out Driver Check
The comparison is made between the modem-out echo and the modem-out
control slots, 2.6 Jls after the setting of the CCITT drivers.
Transmit Data Bit Driver Check (and Control for LIC 4)
A comparison between transmit bit echo and transmit data latch is made at
middle of the data bit.
In X.21, the control (C) echo is checked like the transmit data bit echo.
Chapter 4. Transmission Subsystem (TSS)
4-181