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MT8976

ISO-CMOS ST-BUS™ FAMILY T1/ESF Framer Circuit

Mitel Networks 

MT8976 ISO-CMOS

Bit

Name

.

Description

7 RMLOOP Remote Loopback. When set, the data received at RxA and RxB is looped back to TxB

and TxA respectively. The data is clocked into the device with E1.5i. The device still

monitors the received data and outputs it at DSTo. The device operates normally when the

bit is clear.

6 DGLOOP Digital Loopback. When set, the data input on DSTi is looped around to DSTo. The

normal received data on RxA, RxB and RxD is ignored. However, the data input at DSTi

is still transmitted on TxA and TxB. The device frames up on the looped data using the

C1.5i clock.

5

ALL1'S All One’s Alarm. When set, the chip transmits an unframed all 1's signal on TxA and TxB.

4

ESF/D4 ESF/D4 Select. When set, the device is in ESF mode. When clear, the device is in

D3/D4 mode.

3

ReFR

Reframe. If set for at least one frame and then cleared, the chip will begin to search for a

new frame position. Only the change from high to low will cause a reframe, not a

continuous low level.

2

SLC-96 SLC-96 Mode Select. The chip is in SLC-96 mode when this bit is set. This enables input

and output of the FS bit pattern using the same pins as the facility data link in ESF mode.

The chip will use the same framing algorithm as D3/D4 mode. The user must insert the

valid FS bits in

the transmitter

2 out of 6 superframes to allow the receiver to find superframe sync, and

to insert A and B bits in every 6th frame. The SLC-96 FDL completely

replaces the FS pattern in the outgoing S bit position. Inactive in ESF mode.

1 CRC/MIMIC In ESF mode, when set, the chip disregards the CRC calculation during synchronization.

When clear, the device will check for a correct CRC before going into synchronization. In

D3/D4 mode, when set, the device will synchronize on the first correct S-bit pattern

detected. When this bit is clear, the device will not synchronize if it has detected more than

one candidate for the frame alignment pattern (i.e., a mimic).

0

Maint. Maintenance Mode. When set, the device will declare itself out-of-sync if 4 out of 12

consecutive FT bits are in error. When clear, the out-of-sync threshold is 2 errors in 4 FT

bits. In this mode, four consecutive bits following an errored FT bit are examined.

Table 2. Master Control Word 2 (Channel 31, CSTi0)

schemes to be selected. The three choices are:

none, binary 8 zero suppression (B8ZS), or jammed

bit (bit 7 forced high). No zero code suppression

allows the device to interface with systems that have

already applied some form of zero code suppression

to the data input on DSTi. B8ZS zero code

suppression replaces all strings of 8 zeros with a

known bit pattern and a specific pattern of bipolar

violations. This bit pattern and violation pattern is

shown in Figure 7. The receiver monitors the

received bit pattern and the bipolar violation pattern

and replaces all matching strings with 8 zeros.

Loopback Modes

The digital loop around mode diverts the data

received at DSTi back out the DSTo pin. Data

received on DSTi is, however, still transmitted out via

TxA and TxB. This loop back mode can be used to

test the near end interface equipment when there is

no transmission line or when there is a suspected

failure of the line.

The all one’s transmit alarm (also known as the blue

alarm or the keep alive signal) can be activated in

conjunction with the digital loop around so that the

transmission line sends an all 1's signal while the

normal data is looped back locally.

Remote and digital loopback modes are enabled by

bits 6 and 7 in Master Control Word 2. These modes

can be used for diagnostics in locating the source of

a fault condition. Remote loop around loops back

data received at RxA and RxB back out on TxA and

TxB, thus effectively sending the received DS1 data

back to the far end unaltered so that the

transmission line can be tested. The received signal

is still monitored with the appropriate received

channels on the DS1 side made available in the

proper format at DSTo.

The MT8976 also has a per channel loopback mode.

See Table 6 and the following section for more

information.

Per Channel Control Features

In addition to the two master control words in CSTi0

there are also 24 Per Channel Control Words. These

control words only affect individual DS0 channels.

The correspondence between the channels on CSTi0

and the affected DS0 channel is shown in Fig. 6.

4-36

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