13

IDT77V106L25

Bit 1

Bit 2

Mode

0

0

Normal operating mode

1

0

PHY Loopback

1

1

Line Loopback

Normal Mode

Figure 12 shows normal operating conditions: data to be transmitted is transferred to the TC, where it is queued and formatted for transmission by the PMD.

Receive data from the PMD is decoded along with its clock for transfer to the receiving "upstream system".

PHY Loopback

As Figure 13 illustrates below, this loopback mode provides a connection within the PHY from the transmit PHY-ATM interface to the PHY-ATM receive

interface. Note that while this mode is operating, no data is forwarded to or received from the line interface.

Line Loopback

Figure 14 might also be called "remote loopback" since it provides for a means to test the overall system, including the line. Since this mode will probably

be entered under direction from another system (at a remote location), receive data is also decoded and transferred to the upstream system to allow it to listen

commands. A common example would be command asking the upstream system to direct the TC to leave this loopback state and resume normal operations.

TC sublayer

PMD sublayer

Line

Interface

ATM Layer

Device

77v1054 drw 33

Utopia

Interface

Figure 12. Normal Mode

ATM Layer

Device

TC sublayer

PMD sublayer

Line

Interface

77v1054 drw 34

Utopia

Interface

Figure 13. PHY Loopback

ATM Layer

Device

TC sublayer

PMD

sublayer

Line

Interface

77v1054 drw 35

Utopia

Interface

Figure 14. Line Loopback

Counters

Several condition counters are provided to assist external systems (e.g. software drivers) in evaluating communications conditions. It is anticipated