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MC68194FJ

Carrier Band Modem (CBM)

ON Semiconductor 

MC68194

Figure 1–3. MAC Symbol Encoding

ONE

L

L

ZERO

H

L

ND1

1 BIT TIME =

1/ BIT RATE

L

H

ND2

1 BIT TIME

H

H

Figure 1–4. Phase–Coherent Modulation Scheme

1.3 MESSAGE (FRAME) FORMAT

Although the CBM only uses MAC symbols

one–at–a–time, the MAC or TBC is responsible for

combining the above defined MAC symbols into messages

(more correctly called frames). For the purposes of the

CBM, a simplified frame format can be used consisting of:

SILENCE || PAD–IDLE | START DELIMITER | DATA | END

DELIMITER || SILENCE

where:

PAD–IDLE

= alternating {LL} {HH} pairs which must

occur in octets or groups of eight sym-

bols. Pad–idle provides a training signal

for the receiver and occurs at the begin-

ning of every transmission (and between

frames in a multiple frame transmission).

START = a unique pattern of eight symbols (one oc-

DELIMITER tet) that marks the beginning of a frame.

The pattern is:

ND1 ND2 0 ND1 ND2 0 0 0

where ND1 is the first symbol trans-

mitted.

DATA

= octets of ZERO/ONE patterns that are the

actual data or “information” contained

within the frame.

END

= a unique pattern of symbols that marks

DELIMITER the end of a frame. The pattern is:

ND1 ND2 1 ND1 ND2 1 {I=0/1} {0/1}

where ND1 is the first symbol trans-

mitted. Note that unlike the Start Delimit-

er, the last two bits of the End Delimiter

octet are not always the same. The

seventh bit of the octet is called the I Bit

or Intermediate bit which = 1 when there

is more to transmit and = 0 at the end of

a transmission.

A single transmission can consist of one or more frames.

In a multi–frame transmission, Pad–Idle is sent between

consecutive frames to separate them. If an End Delimiter

occurs within a multi–frame transmission its I Bit will = 1,

and the last end delimiter will have its I Bit = 0.

The CBM accepts a stream of MAC symbols from the

TBC and modulates the phase–coherent transmit signal

accordingly. Conversely, the CBM receives a

phase–coherent signal stream from the cable, decodes the

MAC symbols, and reports them. On transmission there is

a direct one–to–one correlation between MAC symbols

requested and the modulated signal; however, during

reception exceptions can occur. The CBM is allowed to

report Silence or the actual Zero/One pattern during

preamble which is done to allow the receiver to “train” to the

incoming signal. Also, if noise in the system has corrupted

the data, it may show up as an incorrect MAC symbol or the

CBM can report a BAD SIGNAL symbol if an incorrect

combination of ND symbols is detected (ND2 without an

ND1, ND2 followed by ND2, etc.)

1.4 SYSTEM CONFIGURATION

Figure 1–5 illustrates the CBM and peripheral circuitry

required for an IEEE 802.4 carrierband 5 Mbps or 10 Mbps

data rate phase–coherent FSK physical layer. The CBM

communicates with the MAC or TBC through a TTL

compatible serial interface that is consistent with the IEEE

802.4 exposed DTE–DCE interface. Management and

transmission symbol requests are accepted via the CBM

physical data request channel (TXSYM0–TXSYM2,

SMREQ*, and TXCLK). The physical data indication

channel (RXSYM0–RXSYM2, SMIND*, and RXCLK) is

used to send received symbols and management responses

to the MAC.

The periphery circuitry is primarily associated with

interface to the LAN coaxial cable and data recovery. An

external crystal or clock source is required (20 MHz for 5

Mbps data rate or 40 MHz for 10 Mbps data rate) for onboard

timing and transmit clock. Also, an RC timing network sets

the jabber timeout period.

The coaxial cable interface combines the transmit and

receive signal functions. For transmission, the CBM provides

differential drive signals (TXOUT and TXOUT*) whose

[ signaling is ECL levels referenced to VCC (logic high

[ +4.1 V, logic low + 3.3 V) and a gate signal called TXDIS.

The IEEE 802.4 standard puts specific requirements on the

signal transmitted to the cable:

Between +63 dB and +66 dB (1.0 mV, 75 Ω) [dBmV]

output voltage level.

Transmitter–off leakage not to exceed –20 dB

(1.0 mV, 75 Ω) [dBmV].

Signal transition time window (eye pattern)

dependent on data rate.

Because of this, an external amplifier with waveshaping

is required. The CBM TXOUT/TXOUT* outputs provide

complementary signals with virtually no slew, and the

TXDIS is an enable signal helpful for turning the external

amp off “hard” to meet the low level leakage.

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