Application Note

AN-1177

Rev. 0 | Page 3 of 12

BUS TYPES AND TOPOLOGIES

Standard TIA/EIA-644 LVDS devices allow low power, high

speed communication. The advantages of LVDS can also

be applied to multipoint applications by using TIA/EIA-899

devices. Bus topology is one of the main factors relating to

which LVDS or M-LVDS devices are used in an application.

POINT-TO-POINT

Point-to-point bus topologies consist of a single driver and single

receiver connected together using one pair of wires or traces.

Figure 2 demonstrates a typical configuration, where the receiving

end of the link has a termination resistor. This is the most common

application for LVDS devices. Multiple pairs of wires or traces

can be used to create additional channels of communication and

increase total bandwidth between two points.

LVDS

DRIVER

LVDS

RECEIVER

Figure 2. LVDS Point-to-Point Link

Analog Devices, Inc., has a portfolio of LVDS drivers and receivers

for one, two or four LVDS channels as shown in Table 1. Unused

outputs should be left open circuit.

Table 1. LVDS Drivers and Receivers

Part No.

Tx

Rx

Part No.

Tx

Rx

ADN4661

1

0

ADN4665

4

0

ADN4662

0

1

ADN4666

0

4

ADN4663

2

0

ADN4667

4

0

ADN4664

0

2

ADN4668

0

4

M-LVDS can also be used in a point-to-point topology, where the

same transceiver device is used for the driver circuit (with receiver

disabled) and the receiving circuit (with driver disabled).

MULTI-DROP

A single driver can be connected to multiple receivers using

a multi-drop bus topology as shown in Figure 3. LVDS is

designed for point-to-point applications and so in a multi-drop

configuration, the number of receivers that can be connected

and the signaling distance can be limited. M-LVDS can be used

in a multi-drop topology to drive up to 32 nodes across longer

distances compared to LVDS.

LVDS

DRIVER

LVDS

RECEIVERS

Figure 3. LVDS Multi-Drop Bus

MULTIPOINT

In networks where multiple devices can either send or receive,

a multipoint bus topology may be used. M-LVDS is designed

for such multi-point applications, allowing up to 32 nodes to

be connected to a single bus. There are two types of multipoint

buses, half-duplex and full duplex, shown in Figure 4 and

Figure 5, respectively. In a half-duplex bus, two wires are used

such that one device may transmit, and the other devices can

receive. In a full-duplex bus, four wires are used, allowing one

node to concurrently transmit back to another transmitting

node (that is, slave devices responding as broadcast commands

are sent by the master to all nodes).

B

A

B

A

DI

RO

DI

RO

RO

DI

MLVDS

TRANSCEIVERS

Figure 4. M-LVDS Half-Duplex Bus

Z

Y

Z

Y

B

A

B

A

DI

RO

DI

RO

MLVDS

TRANSCEIVERS

RO

DI

RO

DI

Figure 5. M-LVDS Full-Duplex Bus

Another factor to be considered in multipoint buses is the bus

idle condition. When no device is transmitting, the differential

voltage on a terminated bus will be close to 0 V. This means that

for a standard receiver with symmetrical input thresholds, the

receiver output will be undefined. This corresponds to the

Type 1 M-LVDS receivers with an input threshold of ±50 mV.

In order to provide a guaranteed receiver output state (output

low) in the bus idle condition, Type 2 M-LVDS receivers have

an offset receiver input threshold of +50 mV to +150 mV.

Table 2. M-LVDS Transceivers

Part No.

Rx Type

Duplex

Data Rate

ADN4690E

1

Half

100

ADN4691E

1

Half

200

ADN4692E

1

Full

100

ADN4693E

1

Full

200

ADN4694E

2

Half

100

ADN4695E

2

Full

100

ADN4696E

2

Half

200

ADN4697E

2

Full

200