Ref: 13930003
Title: GS/X.25 Clocking Configuration
Date: 11/07/89

Copyright 3Com Corporation, 1991.  All rights reserved.

This topic describes the SIO clocking configuration in the GS/1-XNS/X.25
and the GS/X-IP/X.25.


Configuring the SIO Board

The SIO board has Area E49, E69, and E70, which determine whether the
clock sources are internal or external.  The following table lists shorting
plug settings for the SIO-SMS clock sources:

Port    Clock          Line    E49 Pins    E69 Pins    E70 Pins
No.     Source         Name    Connected   Connected   Connected
----    ------         ----    ---------   ---------   ---------
0       TX External    TXC        1-2*        1-2**       2-3*
0       TX Internal    EXC        2-3         1-2**       1-2
0       RX External    RXC        4-5*        n/a         n/a
0       RX Internal    --         5-6         n/a         n/a

1       TX External    TXC        7-8*        4-5**       5-6*
1       TX Internal    EXC        8-9         4-5**       4-5
1       RX External    RXC       10-11*       n/a         n/a
1       RX Internal    --        11-12        n/a         n/a

            *   Factory default, user-alterable.
           **   Factory default, not user-alterable.

The recommended settings vary according to the type of device being
attached.  In general, for use with modem equipment, both transmit and
receive clock sources should be set to external.  For direct connection,
in general, the transmit clock should be set to internal and the receive
clock to external.

