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LTC2274

2274f

APPLICATIONS INFORMATION

Frame Alignment Monitoring

After the initial synchronization has been established, it may

be desirable to periodically verify that frame alignment is

being maintained. The receiver may issue a synchroniza-

tion request at any time, but data will be lost during the

resynchronization interval.

To verify frame alignment without the loss of data, frame

alignment monitoring is enabled by setting the FAM pin

to a high level. In this mode, predetermined data in the

second code group of the frame is substituted with the

control character K28.7. The receiver is required to detect

the K28.7 character and replace it with the original data. In

this way, the second code group may be discerned from

the ﬁrst, and the receiver is able to periodically verify the

frame alignment without the loss of data (refer to Table 1

and the ﬂow diagram of Figure 18). There are two frame

alignment monitoring modes summarized in Table 1.

FAM mode 1 is implemented when FAM is high, and

SCRAM is low:

• When the data in the second code group of the current

frame equals the data in the second code group of the

previous frame, the LTC2274 will replace the second

code group with the control character K28.7 before

serialization. However, if a K28.7 symbol was already

transmitted in the previous frame, the actual code group

will be transmitted.

• Upon receiving a K28.7 symbol, the receiver is required

to replace it with the data decoded at the same position

of the previous frame.

FAM mode 2 is implemented when FAM is high and

SCRAM is high:

• When the data in the second code group of the current

frame equals D28.7, the LTC2274 will replace this data

with K28.7 before serialization.

• Upon receiving a K28.7 symbol, the receiver is required

to replace it with D28.7.

With FAM enabled the receiver is required to search for

the presence of K28.7 symbols in the data stream. If two

successive K28.7 symbols are detected at the same posi-

tion other than the assumed end of frame, the receiver will

realign its frame boundary to the new position.

Table 1. Frame Alignment Monitoring Modes

SCRAM PIN

DDSYNC PIN

ACTION

FAM Mode 1

Low

High

The second code group is replaced with K28.7 if

it is equal to the 2nd Code Group of the previous

frame

FAM Mode 2

High

The second code group is replaced with K28.7 if

it is equal to D28.7

FAM OFF

Low

No K28.7 substitutions will take place

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