NWK954

7

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.

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.

Fig. 7 Polarity selection on BPDOE_N, BPDIE_N and

BPCOLOE_N

Port status

No incoming signal

Incoming signal but no idle pattern recognized*

Link established, no incoming data packets

Link established, receiving data packets

Link established, but port partitioned due to excessive collisions

or port isolated due to false carriers

or port disabled due to jabber

Link established, false carrier or invalid data detected

RXLED_N

Off

Off

On

Flash

On

Flash off

ERLED_N

Off

Flash

Off

Off

On

Flash on

10k

NWK954

NWK954

Enable signal configured for active low operation

Enable signal configured for active high operation

* 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.

Table 1

LED

ACTLED_N0

ACTLED_N1

ACTLED_N2

ACTLED_N3

ACTLED_N4

Network utilization

>1%

>12·5%

>25%

>50%

>60%

Table 2