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The IRD bus in each hub is connected to the 5-bit backplane
bus via a bidirectional buffer. This allows data to be driven from
any hub to all other hubs in the stack. The bidirectional buffer is
controlled by 2 control signals (BPDOE_N and BPDIE_N) which
can be taken from any one of the NWK954 devices in the hub.
BPDOE_N and BPDIE_N default to being active low but can be
independently changed to active high by the addition of an
external pull-down resistor, as shown in Fig.7. This resistor is
sensed during RESET_N = 0.
Data transfers on the backplane are synchronous to the
backplane clock. The backplane clock is supplied by whichever
hub supplies the data. When receiving data from the backplane,
the backplane clock is enabled onto the local BPCLK signal by
BPDIE_N. BPCLK connects to all NWK954s in the hub. When
a hub supplies data to the backplane, BPCLK is driven locally
and is enabled onto the backplane by BPDOE_N.
Each hub in a stack indicates receive activity on any of its
twisted pair ports by asserting the backplane activity output
(BPACTOUT_N). BPACTOUT_N can be taken from any one of
the NWK954s and drives one of the backplane activity signals
via an external buffer. The backplane supports up to 8 backplane
activity signals. The backplane activity signals from up to 7 other
hubs connect to the local BPACTIN_N signals which are input
to all NWK954s in the hub.
When a collision occurs between 2 twisted pair ports in a
hub, the event is communicated to other stacked hubs by
BPCOL_N which is enabled onto the backplane collision signal
by BPCOLOE_N. BPCOLOE_N defaults to being active low but
can be changed to active high by the addition of an external
pull-down resistor. This resistor is sensed during RESET_N=0.
BPCOL_N is also asserted when a collision occurs between
two twisted pair ports on different hubs. Collision events are
cormmunicated to all hubs through the BPCOLIN_N signal which
connects to all NWK954s in the hub.
RXLED_N
Off
Off
On
Flash
On
Flash off
* In 802.3 a continuous signal of the required amplitude is sufficient to establish a link but no data can be passed to the
repeater controller until the descrambler has locked on to an idle pattern.
ERLED_N
No incoming signal
Off
Incoming signal but no idle pattern recognized*
Flash
Off
Link established, no incoming data packets
Off
Link established, receiving data packets
Link established, but port partitioned due to excessive collisions
On
or port isolated due to false carriers
or port disabled due to jabber
Link established, false carrier or invalid data detected
Flash on
Table 1
LED
ACTLED_N0
ACTLED_N1
ACTLED_N2
ACTLED_N3
ACTLED_N4
Table 2
LED Drivers
The NWK954 provides 2 LED drivers per port to indicate
port status (RXLED_N and ERLED_N), one LED driver to
indicate collisions (COLLED_N) and 5 LED drivers to indicate
network utilization (ACTLED_N). The LED drivers pull the output
pins low to turn the LEDs on. The LEDs are turned on or off for
a minimum of 40ms to ensure observability.
The port status LEDs indicate a variety of conditions and
provide rapid diagnosis of network faults,as shown in Table 1.
The network utilization LEDs are turned on at the thresholds
given in Table 2.
Enable signal configured for active low operation
Enable signal configured for active high operation
Fig. 7 Polarity selection on BPDOE_N, BPDIE_N and
Port status
Network utilization
>1%
>12·5%
>25%
>50%
>60%
NWK954
NWK954
NWK954
1 0 k
BPCOLOE_N
7
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