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MC68194
(Rev.:2006)

Carrier Band Modem (CBM)

ON Semiconductor 

MC68194

SECTION 3

TRANSMITTER

3.1 OVERVIEW

The transmitter function includes the serial interface

decoder, transmit modulator, transmit buffer, jabber inhibit,

and clock generator. (Although the clock generator is not

used exclusively by the transmit function, the generator will

be discussed here.) The MC68194 receives request channel

symbols on the TXSYMX pins which are synchronized to

TXCLK. As is described in the Serial Interface discussion,

MAC transmit symbols are input serially (CBM in MAC

mode), decoded, and used to modulate an output signal. The

Serial Interface Decoder is used both for MAC mode to

decode data transmit commands (symbols) and

management mode to decode management commands. The

decoded transmit commands or symbols are used by the

Transmit Modulator to generate phase−coherent signaling

as discussed in the CBM General Description. The transmit

buffer receives the modulated signal and drives differential

output signals.

The clock generator provides TXCLK and internal clocks

of 2 times (2X) and 4 times (4X) TXCLK. The 4X clock is

actually the oscillator frequency. These clocks are used to

receive the TX symbols and generate the modulated signal.

3.2 TRANSMIT BUFFER

The modulated transmit data stream drives the TXOUT

and TXOUT* pins of the MC68194. These pins are

complementary outputs with closely matched edge

transitions. This is useful in helping meet the IEEE 802.4

carrierband requirement for a transmit jitter of less than

"1% of the data rate. TXOUT and TXOUT* are generally

used to drive a differential amplifier which is used to achieve

the necessary output level at the cable and meet the rise/fall

time window (or “eye” pattern) of the IEEE 802.4. A third

output called TXDIS is available to gate the amplifier

circuitry on or off.

The TXTOUT and TXTOUT* have ECL levels

referenced to VCC (Figure 3−1). Levels are typically 4.11 V

for a high and 3.25 for a low. Pulldown resistors are required

with the outputs specified to drive a maximum load of 220

Ω to ground reference.

Operation of the transmit outputs is controlled in the

following manner:

1. Management mode — The TX outputs are always

disabled while the CBM is in management mode. When

leaving management mode the TX outputs remain

disabled if a RESET command has been issued and an

ENABLE TRANSMITTER and DISABLE

LOOPBACK commands have not been issued.

Resetting the CBM enables internal loopback and

disables the transmitter.

2. MAC (data) mode — After leaving management mode,

the CBM can function in internal loopback (for test) with

the transmitter disabled, out of loopback with transmitter

disabled (receive only), or in standard data mode with the

TX outputs controlled by the modulator.

TXDIS

VCC−TXOUT

TXOUT

TXOUT*

RP TX AMP

RP

Figure 3−1. Transmitter Outputs

3. Jabber inhibit activated — If the jabber inhibit fires, it

forces the CBM into management mode and disables the

TX outputs. This condition can only be cleared by a reset

condition.

The TXDIS output is an open collector switched current

source. TXDIS sinks a nominal 0.5 mA when the

TXOUT/TXOUT* outputs are enabled. TXDIS is off or

high impedance when the transmitter is disabled.

The signaling on the TX outputs and TXDIS is shown in

Figure 3−2. The “off” or silence condition is both TXOUT

outputs high and TXDIS also high. The figure shows an

example of the signal pattern for both leaving and entering

a silence condition.

SILENCE

OFF 1

0

1 ND1 ND2 1 OFF

TXDIS

TXOUT

TXOUT*

Figure 3−2. Transmitter Output Signaling

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