AM79C984 datasheet(20/44 Pages) AMD

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P R E L I M I N A R Y

Am79C984A

Expansion Bus Interface

The eIMR device expansion bus allows multiple eIMR

devices to be interconnected.

The expansion bus supports two modes of operation:

internal arbitration mode and IMR+ mode. The internal

arbitration mode uses a modiﬁed daisy-chain scheme

to eliminate the need for any external arbitration cir-

cuitry. The IMR+ mode maintains the full functionality of

the IMR+ (Am79C981) expansion bus and beneﬁts

from minimum delays. In this mode, the eIMR device

requires external circuitry to handle arbitration for con-

trol of the bus.

The eIMR arbitration mode is determined at reset. This

occurs on the trailing edge of RST according to the

state of SELI0-1, as illustrated in Figure 4.

Internal Arbitration Mode

The internal arbitration mode uses a daisy-chain (cas-

cade) conﬁguration. SELI0-1 are arbitration inputs and

SELO is the arbitration output. SELO goes LOW when

there is activity on one or more of the eIMR ports, or a

SELI input is LOW. The SEL lines are connected as

shown in Figure 5. This technique allows activity indica-

tion to propagate down the chain to the end device. All

unused SELI inputs must be tied to VDD.

ACK and COL are global activity I/O pins. When the

eIMR device senses activity, it drives ACK LOW.

Figure 4. Expansion Bus Mode Selection

An eIMR device drives COL LOW when it senses more

than one device is active; that is, if the device has an

active port AND a SELI input is LOW, OR both SELI in-

puts are LOW. In Boolean notation, the formula for COL

is:

COL = (Active port & (SELI1 + SELI0))+

(SELI1 & SELI0)

where

& represents the Boolean AND operation

+ represents the Boolean OR operation

ACK and COL are mutually exclusive. If an eIMR driv-

ing ACK senses COL LOW, the device will deassert

ACK.

DAT and JAM are synchronized to CLK. DAT is the rep-

etition of data from any connected port (either TP or

AUI port) encoded in NRZ format. JAM is an internal

collision indicator. If JAM is HIGH, the active eIMR de-

vice has detected an internal collision across one or

more of its ports. When this occurs, the DAT signal dis-

tinguishes between single-port collisions and multiport

collisions. DAT = 1 indicates a single port collision;

DAT = 0 indicates a multiport collision.

The drive capabilities of the I/O signals on the expan-

sion bus (DAT, JAM, ACK, and COL) are sufﬁcient to

allow seven eIMR devices to be connected together

without the use of external transceivers or buffers.

The maximum number of eIMR devices that can be

daisy chained is limited by the propagation delay of the

eIMR devices. In practice, the depth of the cascade is

limited to three eIMR devices, thus allowing a maxi-

mum of seven eIMR devices connected together via

this expansion bus as shown in Figure 5.

The active device will not drive the data line, DAT,

until one bit time (100 ns) after SELO goes LOW. This

is to avoid a situation where two devices drive DAT

simultaneously.

IMR+ Mode

In IMR+ mode, the expansion bus requires an external

arbiter. The arbiter allows only one eIMR device to con-

trol the expansion bus. If more than one device at-

tempts to take control, the arbiter terminates all access

and signals a collision condition.

In IMR+ mode, DAT and JAM retain the same function-

ality as in internal arbitration mode, but ACK and COL

are inputs to the eIMR device, driven by the external ar-

biter. The arbiter should drive ACK LOW when exactly

one eIMR device is active. It should drive COL when

more than one eIMR device is active. SELO is an out-

put from the eIMR device. It indicates that the eIMR de-

vice has an active port and is requesting access to the

bus.

When ACK is HIGH, DAT and JAM are in the high-

impedance state. DAT and JAM go active when ACK

goes LOW. Refer to the

Systems Applications section

(Figure 13) for the conﬁguration of IMR+ mode of

operation.

Note: The IMR+ mode is recommended when arbitrating

between multiple boards.

20650B-9

RST

SELI_0

Mode Selection

SELI_1

SELI_0

Arbitration

Mode

Internal

IMR+

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