~ National ~ Semiconductor - Al Kossow's Bitsavers

12.0 Loopback Diagnostics (Continued)
Note 1: CDH is set, CRS is not set since it is generated by the external
encoder/decoder.
Note 1: CDH and CRS should not be set. The TSR however, could also
contain 01 H,03H,07H and a variety of other values depending on whether
collisions were encountered or the packet was deferred.
Note 2.Will contain OSH if packet is not transmittable.
Note 3: During externalloopback the NIC is now exposed to network traffic.
it Is therefore possible for the contents of both the Receive portion of the
FIFO and the RSR to be corrupted by any other packet on the network. Thus
in a live network the contents of the FIFO and RSR should not be depended
on. The NIC will still abide by the standard CSMAlCD protocol in extemal
loopback mode. (i.e. The network will not be disturbed by the loopback
packet).
Nota 4: All values are hex.
CRC AND ADDRESS RECOGNITION
The next three tests exercise the address recognition logic
and CRC. These tests should be performed using internal
loop back only so that the N IC is isolated from interference
from the network. These tests also require the capability to
generate CRC in software.
The address recognition logic cannot be directly tested. The
CRC and FAE bits in the RSR are only set if the address of
the packet matches the address filters. If errors are expected
to be set and they are not set, the packet has been
rejected on the basis of an address mismatch. The following
sequence of packets will test the address recognition logic.
The DCR should be set to 40H, the TCR should be set to
03H with a software generated CRC.
NETWORK MANAGEMENT FUNCTIONS
Network management capabilities are required for maintenance
and planning of a local area network. The NIC supports
the minimum requirement for network management in
hardware, the remaining requirements can be met with software
counts. There are three events that software alone
can not track during reception of packets: CRC errors,
Frame Alignment errors, and missed packets.
Since errored packets can be rejected, the status associated
with these packets is lost unless the CPU can access the
Receive Status Register before the next packet arrives. In
situations where another packet arrives very quickly, the
CPU may have no opportunity to do this. The NIC counts
the number of packets with CRC errors and Frame Alignment
errors. S-bit counters have been selected to reduce
overhead. The counters will generate interrupts whenever
their MSBs are set so that a software routine can accumulate
the network statistics and reset the counters before
overflow occurs. The counters are sticky so that when they
reach a count of 192 (COH) counting is halted. An additional
counter is provided to count the number of packets NIC
misses due to buffer overflow or being offline.
The structure of the counters is shown below:
CIIlRO ,RAME ALIGNMENT ERRORS COUNTER
r+
CIIlRl CRC ERRORS COUNTER
r+ MS8jo-
MS8
INTERRUPT
CNlR2 MISSEO PACI