AM85C30 datasheet(110/194 Pages) AMD

110 / 194 page

Support Circuitry Programming

AMD

5–16

5.6.1

Local Loopback

Local loopback is selected when bit D4 of WR14 is set to ‘1’. In this mode the output of

the transmitter is internally connected to the input of the receiver. At the same time the

TxD pin remains connected to the transmitter. In this mode the DCD pin is ignored as a

receiver enable and the

CTS pin is ignored as a transmitter enable even if the Auto En-

ables mode has been selected. Note that the DPLL input is connected to the

RxD pin, not

to the input of the receiver. This precludes the use of the DPLL in Local Loopback. Local

Loopback is shown schematically in Figure 5–10.

5.6.2

Auto Echo

Auto Echo is selected when bit D3 of WR14 is set to ‘1’. In this mode the

TxD pin is con-

nected directly to the

RxD pin, and the receiver input is connected to the RxD pin. In this

mode the

CTS pin is ignored as a transmitter enable and the output of the transmitter

does not connect to anything. If both the Local Loopback and Auto Echo bits are set to

‘1’, the Auto Echo mode will be selected, but both the

CTS pin and the DCD pin will be

ignored as auto enables. This, however, should not be considered a normal operating

mode. Auto Echo is shown schematically in Figure 5–11.

Receiver

Transmitter

TX Enable

RX Enable

DCD

CTS

RxD

TxD

N.C.

Local Loopback

Figure 5–10. Local Loopback

Receiver

Transmitter

TX Enable

RX Enable

DCD

CTS

RxD

TxD

Auto Echo

N.C.

Figure 5–11. Auto Echo

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