Ref: 23390001
Title: Using an IB/3 With ITC/M Board to Link to an IB/3 With SIO Board
Date: 5/3/91

Copyright 3Com Corporation, 1991.  All rights reserved.

Q:  Can an IB/3 with an ITC/M board communicate across a 56Kbps data
link to another IB/3 with a SIO board?

A:  The IB/3 with an ITC/M board was designed to operate between 128Kbps
and 3.154Mbps (primarily T-1 and Fractional T-1 speeds).  It communicates
with another IB/3 with an ITC/M board or with an NB/IB or NB/BR (with
ItcmCompatible selected on the NB) across a serial link, such as a
synchronous leased line.

Using a IB/3 ITC/M board to communicate with a IB/3 SIO will not work for
two reasons:

1.  The ITC/M board is guaranteeing ones-density by inverting
the data stream in the hardware after performing HDLC Zero
Insertion.  The data is not inverted on an SIO board.

This problem can be circumvented by swapping the pins for TXA and TXB
and swapping the pins for RXA and RXB at the connector which attaches
to the backpanel connector associated with (and located at the IB/3
with the) ITC/M board.

2.  The ITC/M board requires that a 24-bit "preamble" consisting
of alternating ones and zeros be received at the beginning of
each packet; the NB will send an HDLC Flag (7E Hex) instead.  This
problem cannot be circumvented.

Customers have been known to configure, and use, an IB/3 with an
ITC/M board to communicate across a 56Kb data link (to either an
NB/IB with ItcmCompatible selected or to another IB/3's ITC/M board).
Since the ITC/M board has not been certified to operate correctly at
speeds below 128Kbps, this is not supported and therefore it is not
recommended.

