Ref: 02880037
Title: Intel 486 Design Incompatibility Affects Micro Channel Adapters
Date: 11/29/90

Copyright 3Com Corporation, 1991.  All rights reserved.

Because of a design incompatibility between Intel 80486 Micro Channel
computers and 3Com's EtherLink/MC (3C523) and EtherLink/MC TP (3C523-TP)
adapters, 3Com is issuing revised adapters and revised software drivers.

You may have already installed current adapters on your 80486 machines and
noticed no problem.  At worst, systems may hang from time to time and
require restarting.  3Com cannot justify asking you to rush to your 80486
machines to pull out these adapters.  In December 1990, when 3Com ships
the updated versions, we recommend that you exchange your adapters with us
at no charge.

Note:  There is no reason to hesitate in using current adapters for use
in 286- and 386-based machines, which comprise more than 99 percent of the
Micro Channel markets today.  The problem does not occur with any other
adapters or on other types of buses.

The new adapter version will ship with revised NetWare 286, NetWare 386,
and NDIS drivers.  The revised drivers will work with all adapter versions
in all MicroChannel host processor environments.  3Com is working closely
with other software vendors to revise their drivers also.

Following is a technical description of the problem and programming
instructions for revising the drivers.

Current 3Com Micro Channel adapters use the Intel 82586 Ethernet Controller
component.  The 82586 sometimes deasserts the interrupt signal for a short
period of time before the interrupt acknowledge cycle and signal is
produced from the 80486; this is the timing design incompatibility in the
80486.  The solution in the new adapter versions will be added circuitry to
allow latching of the 82586's interrupt signal to keep it asserted, plus
the revision of the software drivers to enable and clear the latch.
EtherLink/MC and EtherLink/MC TP are 100% software compatible, so one
driver will accomodate both.  The driver needs the following modifications:


Enabling the Latch

So that the adapters will work with older drivers on non-80486 machines,
the latch must be initially disabled.  Drivers should be revised to enable
the latch as follows:

  1.  Write 80H to Adapter Revision Level Register (register 7).

  2.  Read Adapter Revision Level Register.

This should be done at adapter initialization time, after initializing
the 82586 but before turning it on.


Clearing the Latch

When an interrupt occurs, the latch must be cleared by the driver
after the driver clears the interrupt in the 82586.  Drivers should be
revised to clear the latch as follows:

  1.  Write 40H to Adapter Revision Level Register (register 7).

  2.  Read Adapter Revision Level Register.

This procedure should be done before the latch is enabled and after the
82586 has been reset by the software.

