MC68194FJ Ver la hoja de datos (PDF) - ON Semiconductor

Número de pieza

componentes Descripción

Fabricante

MC68194FJ

Carrier Band Modem (CBM)

ON Semiconductor 

MC68194

SECTION 1

GENERAL DESCRIPTION

1.1 TOKEN BUS LAN CARRIER BAND NODE OVERVIEW

The MC68194 Carrier Band Modem (CBM) is part of ON

Semiconductor’s solution for an IEEE 802.4 token bus

carrier band Local Area Network (LAN) node. The CBM

integrates the function of the single–channel,

phase–coherent Frequency Shift Keying (FSK) physical

layer. Figure 1–1 illustrates the architecture of a token bus

LAN node as commonly used in Manufacturing Automation

Protocol (MAP) industrial communications. Based on the

ISO–OSI model, the LLC Sublayer and additional upper

layers are typically supported by a local MPU subsystem,

while the IEEE 802.4 token bus MAC Sublayer and Physical

Layer are implemented by the MC68824 Token Bus

Controller (TBC) and MC68194 CBM respectively.

The MC68194 provides the three basic functions of the

physical layer including data transmission to the coax cable,

data reception from the cable, and management of the

physical layer. For standard data mode (also called MAC

mode), the carrier band modem receives a serial transmit

data stream from the MC68824 TBC (called symbols or

atomic symbols), encodes, modulates the carrier, and

transmits the signal to the coaxial cable. Also in the data

mode, the CBM receives a signal from the cable,

demodulates the signal, recovers the data, and sends the

received data symbols to the TBC. Communication between

the TBC and CBM is through a standardized serial interface

inconsistent with the IEEE 802.4 DTE–DCE serial

interface.

MC68000

PROCESSOR

SYSTEM BUS

INTERFACE

MEMORY

LLC

&

UPPER

LAYERS

TOKEN BUS

CONTROLLER

MODULATOR

/

TRANSMITTER

S

T

A DEMODULATOR

TM

/

I G RECEIVER

OM

NT

MAC SUB–LAYER

SERIAL INTERFACE

PHYSICAL

LAYER

MEDIA LAYER

TOKEN BUS COAX

Figure 1–1. IEEE 802.4 Token Bus Carrier Band Node

The physical layer management provides the ability to

reset the CBM, control the transmitter, and do loopback

testing. Also, an onboard RC timer provides a “jabber”

inhibit function to turn off the transmitter and report an error

condition if the transmitter has been continuously on for too

long. Similar to the data mode, the CBM management mode

makes use of the TBC serial interface.

1.2 CARRIER BAND MODULATION TECHNIQUE

The CBM uses phase–coherent frequency shift keying

(FSK) modulation on a single channel system. In this

modulation technique, the two signaling frequencies are

integrally related to the data rate, and transitions between the

two signaling frequencies are made at zero crossings of the

carrier waveform. Figure 1–2 shows the data rate and

signaling frequencies. An {L} is represented as one half

cycle of a signal, starting and ending with a nominal zero

amplitude, whose period is equal to the period of the data

rate, with the phase of one half cycle changing at each

successive {L}. An {H} is represented as one full cycle of

a signal, starting and ending with a nominal zero amplitude

whose period is equal to half the period of the data rate. In

a 5 Mbps implementation, the frequency of {L} is 5.0 MHz

and for {H} is 10 MHz. For a 10 Mbps implementation, the

frequency of {L} is 10 MHz and for {H} is 20 MHz. The

other possible physical symbol is when no signal occurs for

a period equal to one half of the period of the data rate. This

condition is represented by {off}.

Data Rate

MBPS

5

10

Frequency of Lower

Tone MHz {L}

5.0

10

Frequency of Higher

Tone MHz {H}

10

20

Figure 1–2. Data Rate versus Signaling Frequencies

The specified physical symbols ({L}, {H} and {off}) are

combined into pairs which are called MAC–symbols. The

MAC–symbols are transferred across the serial link. The

encodings for the five MAC–symbols are shown in Figure

1–3. Figure 1–4 shows the phase coherent FSK modulation

scheme for ONE, ZERO, and NON–DATA. The IEEE 802.4

document does not specify the polarity used to transmit data

on the physical cable. The receiver must operate without

respect to polarity.

Mac–Symbol

Encoding

Silence

Pad–Idle Pairs

Zero

One

Non–Data

ND1

ND2

{off off}

{L L} {H H}

{H H}

{L L}

{H L}

{L H}

http://onsemi.com

3

Share Link: