connect 3640 and 2621xm back to back via serial?

I am having difficulty connecting these two routers back to back, via a T1 cable. The hardware is a WIC-1DSU-T1= in each device. The problem is, I cannot get the line protocol to come up.

The encapsulation is ppp on both ends
Both ends are using esf framing
Both ends are using a line code of b8zs
Both ends have a clock source of line
the 3640 has the "no-dce-terminal" command
the 2621xm has a clock rate of 4000000
Neither side has alarms that I can see

The cable I made is correct, it works when connecting 2621xm to 2621xm and 3640 to 3640. It does not work when going from 2621xm to 3640.
orusAsked:
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Matt VConnect With a Mentor Commented:
One of those ends needs to provide the clock source.
clock source internal
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sgdoughtConnect With a Mentor Commented:
i usually put the clock rate on both ends for a lab setup.
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Matt VConnect With a Mentor Commented:
Cisco docs all say to set clock rate on one end.  If you set it on both ends you risk going out of sync.
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orusAuthor Commented:
i found the issue. I was using a dual wic on the 2621xm and it didnt synch up right to the 3640. Once both ends used a single t1 card, it worked

thanks
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