TL/F/12034
Loopback
Diagnostics
Using
the
DP8390/901/902/905
AN-937
National Semiconductor
Application Note 937
Bonnie Wilson
July 1994
Loopback Diagnostics
Using the DP8390/
901/902/905
1.0 OVERVIEW
Loopback capabilities are provided on the DP8390/901/
902/905 to perform certain tests in order to validate opera-
tion of the device prior to transmission and reception of
packets on a live network. Typically these tests may be per-
formed during power up and initialization of a node.
This document describes the different loopback modes and
their operation in the DP8390/901/902/905. There are sev-
eral restrictions during loopback that are also discussed.
Detailed instructions on how to generate different loopback
tests are given, as well as the actual code that generates
the tests (Appendix A). This document also discusses varia-
tions in loopback results caused by several common config-
uration errors.
Throughout this document, the term NIC refers to the con-
troller, SNI refers to the ENDEC, CTI refers to the Coaxial
Transceiver Interface, and TPI refers to the Twisted Pair
Interface. These devices comprise the NIC chip set. The
NIC family consists of the DP8390, DP83901, DP83902, and
DP83905.
The loopback diagnostics verify:
1. The integrity of the data path through each block; re-
ceived data is checked against transmitted data.
2. The CRC logic’s capability to generate good CRC on
transmit.
3. The CRC checking capability of the NIC on receive.
4. The address recognition logic’s ability to accept packets
that have a matching address and reject packets that fail
to match an address.
TL/F/12034 – 1
FIGURE 1. Loopback Mode 1: Through the Controller Module
C
1995 National Semiconductor Corporation
RRD-B30M75/Printed in U. S. A.
