12.0 Loopback Diagnostics
(Continued)
3. Verify that the Address Recognition Logic can
a) Recognize address match packets
b) Reject packets that fail to match an address
LOOPBACK OPERATION IN THE ST-NIC
Loopback is a modified form of transmission using only half
of the FIFO. This places certain restrictions on the use of
loopback testing. When loopback mode is selected in the
TCR, the FIFO is split. A packet should be assembled in
memory with programming of TPSR and TBCR0, TBCR1
registers. When the transmit command is issued the follow-
ing operations occur:
Transmitter Actions
1. Data is transferred from memory by the DMA until the
FIFO is filled. For each transfer TBCR0 and TBCR1 are
decremented. (Subsequent burst transfers are initiated
when the number of bytes in the FIFO drops below the
programmed threshold.)
2. The ST-NIC generates 56 bits of preamble followed by an
8-bit synch pattern.
3. Data transferred from FIFO to serializer.
4. If CRC
e
1 in TCR, the CRC is not calculated by ST-NIC,
and the last byte transmitted is the last byte from the
FIFO (Allows software CRC to be appended). If CRC
e
0, ST-NIC calculates and appends four bytes of CRC.
5. At end of Transmission PTX bit set in ISR.
Receiver Actions
1. Wait for synch, all preamble stripped.
2. Store packet in FIFO, increment receive byte count for
each incoming byte.
3. If CRC
e
1 in TCR, receiver checks incoming packet for
CRC errors. If CRC
e
0 in TCR, receiver does not check
CRC errors, CRC error bit always set in RSR (for address
matching packets).
4. At the end of receive, the receive byte count is written
into the FIFO, and the receive status register is updated.
The PRX bit is typically set in the RSR even if the address
does not match. If CRC errors are forced, the packet
must match the address filters in order for the CRC error
bit in the RSR to be set.
EXAMPLES
The following examples show what results can be expected
from a properly operating ST-NIC during loopback. The re-
strictions and results of each type of loopback are listed for
reference. The loopback tests are divided into two sets of
tests. One to verify the data path, CRC generation and byte
count through all three paths. The second set of tests uses
internal loopback to verify the receiver’s CRC checking and
address recognition. For all of the tests the DCR was pro-
grammed to 40H.
Path
TCR RCR
TSR
RSR
ISR
ST-NIC Internal
02
1F
53
02
02
(Note 1) (Note 2) (Note 3)
Note 1:
Since carrier sense and collision detect are generated in the EN-
DEC module, they are blocked during NIC loopback. Carrier and CD
heartbeat are not seen and the CRS and CDH bits are set.
Note 2:
CRC errors are always indicated by the receiver if CRC is appended
by the transmitter.
Note 3:
Only the PTX bit in the ISR is set, the PRX bit is only set if status is
written to memory. In loopback this action does not occur and the
PRX bit remains 0 for all loopback modes.
Note 4:
All values are hex.
Path
TCR
RCR
TSR
RSR
ISR
ST-NIC Internal
04
1F
43
02
02
(Note 1)
Note 1:
CDH is set, CRS is not set since it is generated by the external
encoder/decoder.
Path
TCR
RCR
TSR
RSR
ISR
ST-NIC External
06
1F
03
02
02
(Note 1)
(Note 2)
Note 1:
CDH and CRS should not be set. The TSR however, could also
contain 01H, 03H, 07H and a variety of other values depending on
whether collisions were encountered or the packet was deferred.
Note 2:
The ISR will contain 08H if packet is not transmittable.
Note 3:
During external loopback the ST-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 ST-NIC will still
abide by the standard CSMA/CD protocol in external loopback
mode. (i.e., The network will not be disturbed by the loopback pack-
et.)
Note 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
loopback only so that the ST-NIC is isolated from interfer-
ence from the network. These tests also require the capabil-
ity 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 in
the packet matches the address filters. If errors are expect-
ed 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 and the TCR should be set
to 03H with a software generated CRC.
Packet Contents
Results
Test
Address
CRC
RSR
Test A
Matching
Good
01 (Note 1)
Test B
Matching
Bad
02 (Note 2)
Test C
Non-Matching
Bad
01
Note 1:
Status will read 21H if multicast address used.
Note 2:
Status will read 22H if multicast address used.
Note 3:
In test A, the RSR is set up. In test B the address is found to match
since the CRC is flagged as bad. Test C proves that the address
recognition logic can distinguish a bad address and does not notify
the RSR of the bad CRC. The receiving CRC is proven to work in
test A and test B.
Note 4:
All values are hex.
NETWORK MANAGEMENT FUNCTIONS
Network management capabilities are required for mainte-
nance and planning of a local area network. The ST-NIC
supports the minimum requirement for network manage-
ment 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.
39
Obsolete
