DP83953 datasheet(27/90 Pages) NSC

27 / 90 page

4.0 Functional Description (Continued)

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4.5 Description Of Hardware Connection For In-

ter-ric Bus

When considering the hardware interface, the Inter-RIC

bus may be viewed as consisting of three groups of signals:

1. Port Arbitration chain, namely: ACKI and ACKO.

2. Simultaneous drive and sense signals, i.e., ACTN and

ANYXN. (Potentially these signals may be driven by mul-

tiple devices).

3. Drive or sense signals, i.e., IRE, IRD, IRC and COLN.

(Only one device asserts these signals at any instance in

time).

The first set of signals are either used as point to point

links, or with external arbitration logic. In both cases the

load on these signals will not be large, so the on-chip driv-

ers are adequate. This may not be true for signal classes

(2) and (3).

The Inter-RIC bus has been designed to connect RIC2As

together directly, or via external bus transceivers. The latter

is advantageous in large repeaters. In the second applica-

tion the backplane is often heavily loaded and is beyond

the drive capabilities of the on-chip bus drivers. The need

for simultaneous sense and drive capabilities on the ACTN

and ANYXN signals, and the desire to allow operation with

external bus transceivers, makes it necessary for these bus

signals to each have a pair of pins on the RIC2A. One

drives the bus, the other senses the bus signal. When

external bus transceivers are used, they must be open col-

lector / open drain to allow wire-ORing of the signals. Addi-

tionally, the drive and sense enables of the bus transceiver

should be tied in the active state.

The uni-directional nature of information transfer on the

IRE, IRD, IRC and COLN signals, means a RIC2A is either

driving these signals or receiving them from the bus, but

not both at the same time.

Thus a single bi-directional

input / output pin is adequate for each of these signals. If

an external bus transceiver is used with these signals the

Packet Enable "PKEN" RIC2A output pin performs the

function of a drive enable and sense disable.

Figure 10 shows the RIC2A connected to the Inter-RIC bus

via external bus transceivers, such as National's DS3893A

bus transceivers.

Figure 10. External Bus Transceiver Connection Diagram

Figure 11. Mode Load Operation

Note: DE = Bus Drive Enable active high, /RE = Bus Receive Enable active low

Note: The Inter-RIC bus is configured to use active low signals.

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