ISO-CMOS

MT8979

4-177

Table 16. CRC Error Count: Data Format for CSTo Channel 20

Table 17. Master Status Word 2 (MSW2): Data Format for CSTo Channel 21

BIT

NAME

DESCRIPTION

7 - 0

CERC

CRC Error Counter: This byte is the CRC error counter. The counter will wrap around

once it reaches FF count. If maintenance option is activated, the counter will reset after a

one second interval.

BIT

NAME

DESCRIPTION

7Si2

The received Si bit in frame 15 is reported in this bit. Si2 will be updated after each

RxMF pulse (pin 23).

6Si1

The received Si bit in frame 13 is reported in this bit. Si1 will be updated after each

RxMF pulse (pin 23).

5-4

NA

Unused.

3

CRCTimer

CRC Timer: Transition from 1 to 0 indicates the start of one second interval in which

CRC errors are accumulated. This bit stays high for 8 ms.

2

CRCRef

CRC Reframe: A ’1’ indicates that the receive CRC multiframe synchronization could

not be found within the time out period of 8 ms after detecting frame synchronization.

This bit will go low if CRCSync goes low or if Maintenance is not activated.

1

CRCSync

CRC Sync: A ’0’ indicates that CRC multiframing has been detected.

0

FrmPhase

Frame Count: This is the ninth and most significant bit (b8) of the Phase Status Word

(see Table 15). If the phase status word is incrementing, this bit will toggle when the

phase reading exceeds ST-BUS channel 31, bit 7. If the phase word is decrementing,

then this bit will toggle when the reading goes below ST-BUS channel 0, bit 0.

Applications

The MT8979 is only a link interface to the CEPT

trunk. As such, an external line driver and receiver is

required along with an appropriate pulse transformer

before being connected to the line.

Transmitter

In order to generate a bipolar line signal, the link

interface to the MT8979 provides the user with two

bipolar steering outputs, TxA and TxB.

These

correspond to the required positive and negative

pulses on the transmission line. Figure 14 shows a

recommended output circuit for driving a line pulse

transformer.

The transistors are driven into saturation when they

are turned on, which applies a step function to the

transformer.

The step input to the transformer

produces a nearly constant di/dt before the current

reaches steady state. By operating in the transient

portion of the inductance response, the secondary of

the transformer produces an almost square pulse.

The base terminal of the transistors is AC coupled to

ground.

Figure 14 - Bipolar Line Driver

TxA

TxB

MT8979

•

•

•

•

•

•

•

•

•

+12V

33

µH

47µF

•

TIPo

RINGo

:.5

1.:

1.: