Verilink AS100 (896-502379-001) Product Manual - Manual (Page 109)

Access System Centum Series User Manual C-5
Cables for Internal/External Timing
Please refer to Worksheet items #12 and #18 on page 3-2.
Internal/External timing is supported with standard straight-through
cabling. However, as shown below (V.35) and on the next page
(RS-422A), the attached DTE must internally loop back its incoming
Transmit Clock to its outgoing External Transmit Clock. If this can not
be done inside the DTE itself, the same function can be provided with
cable jumpers. Contact factory if assistance is needed.
IMPORTANT. If Int/Ext has been chosen because because os excessive
DTE cable length and cable jumpers are installed, the indicated jumpers
must be as physically close to the customer DTE as is possible
(preferably inside the DTE connector).