AM79C985 datasheet(32/48 Pages) AMD

32 / 48 page

Am79C985

PRELIMINARY

Figure 11.

RXD Termination

MAC Interface

The eIMR+ device can be connected directly to a MAC

through the AUI port. This requires that the AUI port be

conﬁgured in the reverse mode and connected as

shown in Figure 12a. Notice that DI is connected to DO

of the MAC and DO is connected to DI of the MAC, be-

cause the reverse conﬁguration only affects CI. Where

CI is an input in the normal mode, in the reverse mode,

CI is an output. Figure 12b shows the normal AUI con-

ﬁguration for reference.

Figure 12.

AUI Port Interconnections

Internal Arbitration Mode Connection

The internal arbitration mode uses a modiﬁed daisy-

chain scheme to eliminate the need for any external

arbiter. In this mode, ACK and COL need to be pulled

up through a minimum resistance of 1 k

Ω. The DAT and

JAM pins also need to be pulled down via a high value

resistor. Refer to Figure 13.

IMR+ Mode External Arbitration

The IMR+ mode maintains the full functionality of AMD’s

IMR+ (Am79C981) device’s expansion bus. In this

mode, the eIMR+ device requires external circuitry to

handle arbitration for control of the bus. Figure 14 shows

the conﬁguration for the IMR+ mode of operation.

20650A-14

1:1

Twisted Pair

100

Ω

100

Ω

RXD+

RXD–

20650A-14

20651B-16

DO+

DO–

DI+

DI–

CI+

CI–

DI+

DI–

DO+

DO–

CI+

CI–

DI+

DI–

DO+

DO–

CI+

CI–

DI+

DI–

DO+

DO–

CI+

CI–

a) Reverse Mode (with MAC)

b) Normal Mode (with MAU)

Am79C940

eIMR+

Am7996

eIMR+

0.1

µF

0.1

µF

0.1

µF

0.1

µF

39 – 150

Ω

0.1

µF

Ω

–9 V

1:1

20651A-17

20651B-17

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