MB86967 Datasheet, PDF (30/129 Pages) Fujitsu Component Limited. – LAN Controller with PC Card, ISA Bus, and General-purpose Bus Interfaces

MB86967
8.5 Time Domain Reflectometry
When a node transmits, a short or open on the network causes a reflected signal to the node receiver, which
can sometimes be detected. The reflection causes failure of the carrier sense or detection of a false collision.
An open on the network may cause a false collision, whereas a short usually causes loss of carrier sense. Time
domain reflectometry (TDR) allows estimates of the distance along the network cable from the node to the fault.
The MB86967 is equipped with a special counter to perform the TDR function. The contents of the counter after
any transmission can be determined by reading the Time Domain Reflectometry registers, DLCR14 (the least-
significant byte) and DLCR15 (the most-significant byte). Only the lower 14 bits of the counter are equipped,
which is more than is needed for an IEEE or Ethernet LAN. The top two bits, DLCR15<7:6>, are always 0. The
TDR counter counts the actual number of bits transmitted for each packet before a collision indication, carrier
loss indication or completion of transmission, whichever comes first. A complete transmission with no error
indications clears the TDR counter. The elapsed time represents twice the signal delay from node to fault.
To perform the TDR fault test, first enable interrupts for TMT OK, by setting DLCR2<7> high. An alternative to
using the interrupt is to poll the TMT OK bit, looking for a high level. Set the 16 Collisions Register, BMPR11,
to 07H for this test (no halt, skip-failed packet). Clear status bits by writing 0FF86H to the Receive and Transmit
Status registers. Next, try to transmit a packet length of 600 or more bits. Up to 16 attempts may be made
automatically, if collisions are indicated. Upon completion of the transmission attempts, TMT OK goes high,
generating an interrupt if so enabled. When this occurs, read the Transmit Status register and the TDR register.
8.6 Interpreting the Results
If the count is zero, no fault was detected. If the count is greater than zero, but smaller than the packet length,
a cable fault may exist. If the count is less than 525, a real collision may have occurred during the test. Real
collisions normally occur within the first 65 bytes of the packet, including preamble. Note the error status bits,
COL and CR LOST. COL high suggests a cable open, whereas CR LOST suggests a short. Repeat the
measurement several times, discarding any anomalous values, and average the rest. A cluster of readings at
about the same value is a strong indicator of a valid fault measurement. If such a cluster of readings occurs,
multiply the average of the cluster by 39 feet to estimate the distance from the node to the fault. [39 feet = (100
ns × 0.8 × 186,282 miles/second × 5280 feet/mile)/2; this assumes the network is mostly coaxial cable with
signal propagation speed of approximately 0.8 × C, the speed of light.]
9. Receiver Circuits
The receiver includes a receive state machine, serial-to-parallel conversion, pipe-line FIFO, preamble
recognition, bit and byte-framing, address filtering, CRC and other error checking. Additional circuits involved
in packet reception are described in the Transceiver section of this document.
The receiver state machine provides sequencing of events for the receiver, including idle, busy, address filtering,
and data storage, detects receive error conditions and sets appropriate bits within the DLC registers. A small
data FIFO provides elastic buffering for synchronization with the buffer controller timing and buffering of data
while the buffer controller is servicing other buffer memory access requests.
9.1 Monitoring the Network
Whenever the data link section is enabled (ENA DLC bit, DLCR6<7>, is set to zero), the MB86967 constantly
monitors the network for carrier. Signals that exceed the AC and DC squelch thresholds of the transceiver cause
the internal carrier sense line to assert, which in turn causes the receiver to attempt to receive a packet. (The
transmitter also uses the carrier sense function to defer to transmissions from other nodes.)
The receiver monitors the serial data stream from the transceiver for the end-of-preamble bit pattern, a four-bit
pattern of 1011 ending the preamble’s pattern of alternating ones and zeros. This pattern also provides byte
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