XRT4500CV datasheet(51/99 Pages) EXAR

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XRT4500

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MULTIPROTOCOL SERIAL NETWORK INTERFACE IC

REV. 1.0.7

2. The Analog/Line-Side Loop-back path:

This loop-back path is referred to as an “Analog/Line-

Side” Loop-back, because all signals originate from and

are ultimately terminated by the DCE Terminal. These

signals originate from the DCE Terminal; and are out-

putted to the line, to the DTE Terminal. However, these

signals (from the DCE Terminal) are never converted in-

to the Digital format (by the DTE Mode XRT4500).

These signal are kept in the “Analog” format, and are

looped-back (over the line) to the DCE Terminal.

The RXD signal (originating from the DCE Terminal)

will be transmitted over the line to the DTE Terminal.

However, this signal will not be converted into the dig-

ital format by the “DTE Mode” XRT4500. Instead, this

signal will be looped-back out to the “DCE Terminal”

via the “TXD” signal path.

NOTE: In this loop-back mode, the RXC signal (e.g., the

companion clock signal to RXD) is also received by the

DTE Terminal and looped-back out to the DCE Terminal. In

this case, the “RXC” (Receive Clock) signal will be routed to

the DCE Terminal through the “SCTE” signal path The DCE

SCC can still use the RXC (via the SCTE signal path), in

order to sample the RXD signal (which is available via the

“TXD” signal path).

Behavior of the DCE Mode XRT4500, when the

Loop-Back Mode is Enabled.

Figure 19 presents an illustration of a DTE and a

DCE Terminal interfaced to each other. In this case,

the XRT4500 (associated with the DCE Terminal) has

been configured to operate in the “Loop-back” Mode.

NOTE: Figure 19 only depicts the “High-Speed” DCE/DTE

Interface signals. The “Low-Speed” control/handshaking

signals are not affected by the loop-back mode.

If the Loop-back Mode is configured within the

XRT4500, while it is operating in the DCE Mode, then

the following two (2) loop-back paths exists.

• A Digital/Terminal-Side Loop-back

• An Analog/Line-Side Loop-back

Each of these Loop-back paths are described below.

1. The Digital/Terminal Side Loop-back:

Again, this loop-back path is referred to as a “Digital/

Terminal Side” Loop-back, because all of the signals

originate from, and are terminated by the DCE SCC

(e.g., the Terminal Equipment). The signals (originat-

ing at the DCE SCC) are not converted into the Ana-

log format, and are not output to the line.

The “RXD” signal (originating from the DCE SCC)

along with the “RXC” (Receive Clock) signal will not

be converted into the Analog format, nor output to the

DTE Terminal (over the line). Instead, this signal will

remain in the “Digital-format” and will be looped-back

into the DCE SCC. The “RXD” signal will ultimately be

output to the DCE SCC via the “TXD” output of the

“DCE Mode” XRT4500.

NOTE: The “RXC” signal (e.g., the companion clock signal to

“RXD”) will also be loop-back into the “DCE SCC”. This signal

will be output (by the XRT4500) via the “SCTE” output pin.

FIGURE 19. ILLUSTRATION OF THE BEHAVIOR OF THE DCE MODE XRT4500, WHEN IT IS CONFIGURED TO OPERATE

IN THE

LOOP-BACK MODE

SCC (R)

SCC (L)

XRT4500

RX1

TX1

RX2

TX2

RX3

TX3

RX2

TX2

RX1

TX1

RXD

RXC

TXC

SCTE_IN

TXD_IN

TXD

SCTE

TXC_IN

RXC_IN

RXD_IN

TXD

SCTE

TXC

RXC

RXD

Analog/Line

Loop-back Path

Digital/Terminal

Loop-back Path

DTE (#1)

DCE (#4)

MUX 1

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