MB86967 Datasheet, PDF (29/129 Pages) Fujitsu Component Limited. – LAN Controller with PC Card, ISA Bus, and General-purpose Bus Interfaces

MB86967
packets are loaded into the transmit buffer, the system turns the transmitter on to initiate transmission. Observing
the media access protocol, the MB86967 defers transmitting to carrier from other nodes, minimum interpacket
gap intervals and backoff intervals, if any, and then begins to serialize the data. It generates the Preamble field
at the beginning, and calculates and appends the CRC field at the end. Figure 7 illustrates the standard packet
format. The serialized signal is routed to the transceiver section which encodes the data into the required
Manchester code, appends an end-of-packet delimiter.
The transmitter transmits the packets in the transmit buffer in the order in which they were loaded. If a collision
is detected by the transceiver, the transmitter automatically retransmits the packet until successful or until 16
consecutive attempts have ended in collision. In the latter case, depending on the mode selection made at
initialization time, the transmitter continues to try to transmit the same packet starting again with a collision count
of zero, skips the current packet and tries to transmit the next packet starting with a collision count of zero, or
halts and waits for instruction from the host. In the last case, the host can elect to terminate transmission attempts
by setting ENADLC, DLCR6<7> to one, continue to attempt to transmit the same packet (collision counter reset),
or skip the current packet and try to transmit the next packet(collision count is zero).
DESTINATION
LENGTH
ADDRESS _SOURCE
PREAMBLE
ADDRESS
DATA
8
6
6
2
46-1500
LENGTHS SHOWN IN BYTES
Figure 7 Packet Format
CRC
4
8.3 Backoff Generator
A 17-bit pseudo-random number generator clocked at the 10 MHz bit rate provides the collision backoff function.
Distances between stations become part of the randomizing function. The number generator is sampled at the
time of collision, masking all but the appropriate number of bits specified by the 8802-3 backoff algorithm. This
value is then counted down at the slot-time rate (512 bit times) to generate the backoff interval. For a first collision,
only one bit is used, giving a backoff of either 51.2 microseconds or 0. For a second consecutive collision, two
bits are used, and so forth, up to ten bits. From the tenth to the 16th collisions, 10 bits are used. This generates
a pseudo-random backoff interval of from 0 to 52.38 ms, the so-called ‘binary exponential backoff’ for collisions
per the 8802-3 standard.
8.4 Transmit Error Processing
The MB86967 provides four transmit error status bits in its Transmit Status Register (DLCR0) for reporting the
four possible transmit errors. The errors are: 1) loss of carrier during transmission, which usually indicates a
medium fault or a collision, 2) collision, 3) 16 consecutive collisions and 4) jabber error, which occurs if the length
of a single transmission substantially exceeds the time required to transmit a maximum length packet conforming
to the standard. The latter three can be enabled separately to generate interrupts.
A status bit in the Transmit Status Register is set in case a collision terminates transmission. Collision counter
DLCR4<7:4>, automatically increments after each collision up to the sixteenth collision, at which time it rolls
over to zero. Another status bit indicates that sixteen consecutive attempts to transmit a packet have been made
and all have been terminated by collision. The occurrence of 16 collisions may indicate a network problem, such
as a disconnected cable or terminator, that produces false collisions. While rare, 16 collisions may normally occur.
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