Black Box 5000 series Reference Manual page 53

Menus Reference Manual: ISDN Set-Up Menu
4/5/6/7/8 - Loop compensation
The Loop Compensation option inverts the transmit clock signal generated by the interface module. This may be required
to compensate for phase shifts between the clock signal and transmitted data signal; a problem that may show up as a large
number of CRC errors when attempting to operate the connection at speeds greater than 64 Kbps.
Default: [disabled]
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