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12.0 Loopback Diagnostics (Continued)
Alignment of the Received Packet in the FIFO
Reception of the packet in the FIFO begins at location zero,
after the FIFO pointer reaches the last location in the FIFO,
the pointer wraps to the top of the FIFO overwriting the
previously received data. This process continues until the
last byte is received. The NIC then appends the received
byte count in the next two locations of the FIFO. The contents
of the Upper Byte Count are also copied to the next
FIFO location. The number of bytes used in the loopback
packet determines the alignment of the packet in the FIFO.
The alignment for a 64-byte packet is shown below.
FIFO
FIFO
LOCATION CONTENTS
LOWER BYTE COUNT
UPPER BYTE COUNT
UPPER BYTE COUNT
LAST BYTE
CRCl
CRC2
CRC3
CRC4
-+ First Byte Read
Second Byte Read
Last Byte Read
For the following alignment in the FIFO the packet length
should be (N x 8) + 5 Bytes. Note that if the CRC bit in the
TCR is set, CRC will not be appended by the transmitter. If
the CRC is appended by the transmitter, the last four bytes,
bytes N-3 to N, correspond to the CRC.
FIFO
LOCATION
LOOPBACK TESTS
TestlngCRC
FIFO
CONTENTS
BYTEN-4
BYTE N·3 (CRC1)
BYTE N·2 (CRC2)
BYTE N·l (CRC3)
BYTE N (CRC4)
LOWER BYTE COUNT
UPPER BYTE COUNT
UPPER BYTE COUNT
AR
First Byte Read
Second Byte Read
Las1 Byte Read
If CRC = 0 in the TCR, the NIC computes and appends a
4-byte FCS field to the packet as in normal operation. The
CRC will not be verified during reception in loopback mode.
The CRC must be read from the FIFO and verified by comparison
to a previously computed value.
If CRC = 1, the NIC will not append a 4-byte FCS field. The
user must supply a pre-calculated CRC value and append it
to the transmitted packet.
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
cannot 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. 8-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:
CHTRO FRAME ALIGNMENT ERRORS COUNTER
I-
CNTRl CRe ERRORS COUNTER !.ISB
I-
CNTR2 MISSED PACKETS COUNTER
I-
WSBjD-
"SB
INTERRUPT
TlIF/9345-23
Additional information required for network management is
available in the Receive and Transmit Status registers.
Transmit status is available after each transmission for information
regarding events during transmission.
Typically, the following statistics might be gathered in software:
Traffic:
Errors:
Frames Sent OK
Frames Received OK
Multicast Frames Received
Packets Lost Due to Lack of Resources
Retries/Packet
CRC Errors
Alignment Errors
Excessive Collisions
Packet with Length Errors
Heartbeat Failure
1-80