MB86960 Datasheet, PDF (38/65 Pages) Fujitsu Component Limited. – NETWORK INTERFACE CONTROLLER with ENCODER/DECODER (NICE)

MB86960
first 2/3 of the interval, if for some reason carrier
reappears on the network, NICE resets its interval timer
to re-time the interval from the end of the new
transmission. Such an event can occur during a collision,
since data and carrier indications can be corrupted by the
superimposition of the two packets. During the last 1/3 of
the interval, NICE will ignore a carrier indication if it
occurs. This is in accordance with 8802–3, intended to
assure fairness and equality in access to the network. If
one station starts to transmit slightly ahead of another, no
advantage will be gained by the slightly earlier start. Both
nodes will transmit, a collision will occur, and the
media-access contention will be resolved by backoff
interval differentials.
Data Encoder
NICE serializes the data for transmission, and converts
each bit to Manchester Code, the format used on the
network media. Manchester Code for ‘1’ is a 100 ns
interval starting with a low, ending with a high, with a
low-to-high transition at the 50 ns point. Manchester
Code for ‘0’ is the inverse of this. See Figure 14 for a
block diagram of the encoder/decoder section.
Operating with an External Encoder/Decoder
An option is provided on NICE to disable the on-chip
encoder/decoder circuits and use an external encoder/de-
coder. Specific details are given in the register description
section in Table 13, Configuration Register 1 (see
DLCR7<7:6>).
Transmit Packet Processing
To transmit one or more packets, the host system first
loads the packet(s), preceded by a 2-byte header giving
their lengths, into a transmit buffer by writing the data to
the Buffer Memory Port Register, BMPR<8:9>. Only the
destination address, source address, length field and
data field of the packets are loaded by the system, NICE
generates the rest. When the packets are loaded, the
system turns the transmitter on to initiate transmission.
This enables NICE to transmit. Observing the media
access protocol, NICE defers transmitting to carrier from
other nodes, minimum inter-packet gap intervals and
backoff intervals if any, then begins to transmit. NICE
serializes the data and encodes it in Manchester Code. It
generates the preamble field at the beginning, and
calculates and appends the CRC field at the end, followed
by the End-of-Packet Delimiter, which is a non-Man-
chester code. The Manchester-encoded signals are output
through a differential driver to the TX DATA± pins to the
external transceiver.
The driver is capable of driving a 50-meter segment of
78-Ohm transceiver cable, as specified in the 8802–3
standard. 270-Ohm resistors to GND are required
externally to pull down TXDATA+ and TXDATA–. See
Figures 11 and 12 for suggested cable interfacing.
The host system activates the transmitter by writing 1 to
the TX START bit and the packet count to TX PKT CNT.
After this is done, the transmitter will transmit each packet
in the buffer in the order that they were loaded. If a
collision occurs, 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 will either 1) continue to try to
transmit the same packet, starting again with a collision
count of zero, 2) skip the current packet and try to
transmit the next packet, starting with a collision count
of zero, or 3) halt and wait for instruction from the host. In
the third case the host can select to either 1) terminate by
setting DLC EN high, 2) continue to attempt to transmit
the same packet (collision counter gets reset) or 3) skip
the current packet and try to transmit the next packet
(collision count = 0).
NICE
COL+
COL
39Ω
0.1mF
39
RXDATA+
RXDATA
39Ω
0.1mF
39Ω
TXDATA+
270Ω
TXDATA
270Ω
NoteT:ransformers are 1:1 and 27 mH minimum.
a. IEEE 802.3 AUI (Transceiver) Cable Termination
Figure 11. Transceiver Interface Termination