Table of Contents
MLVDS Design Guide
This document is intended to provide a quick reference and introduction to Multipoint Low Voltage Differential Signalling (Multipoint LVDS or M-LVDS). In addition to introducing MLVDS, overviews will be given of relevant topics relating to MLVDS bus implementations.
The standard TIA/EIA-899 for M-LVDS extends TIA/EIA-644 LVDS (Low Voltage Differential Signalling) to address multipoint applications. M-LVDS allows higher speed communication links than TIA/EIA-485 (RS-485) with lower power.
Additional features of MLVDS over LVDS include:
- Increased driver output strength
- Controlled transition times
- Extended common-mode range
- Option of failsafe receivers for bus idle condition
MLVDS is used for high-speed backplane, cabled and board-to-board data transmission and clock distribution, as well as communication links within a single PCB.
Why use M-LVDS?
M-LVDS has low power requirements and is characterized by differential signaling with a low differential voltage swing. M-LVDS specifies a slightly greater differential output voltage than LVDS in order to allow for the increased load from a multipoint bus. MLVDS is also designed for high-speed communication. Typical applications utilize PCB traces or short wired/backplane links. The common mode range of LVDS was designed for these applications. MLVDS has an extended common mode range compared to LVDS to allow for the additional noise in a multipoint topology. Both the differential output voltage and common mode range specifications of LVDS and MLVDS are shown in the image on the right.
Bus Topologies for M-LVDS
Although the primary application for M-LVDS is in multi-point bus topologies, M-LVDS drivers and receivers (transceivers) can be used in other bus topologies, such as point-to-point and multi-drop topologies as an alternative to LVDS.
Point-to-point bus topologies consist of a single driver and single receiver connected together using one pair of wires or traces. The image below demonstrates a typical configuration, where the receiving end of the link has a termination resistor. This is the most common application for LVDS devices, but MLVDS transceivers can also be used for point-to-point applications, for example if the transmission distance is longer. Multiple pairs of wires or traces can be used to create additional channels of communication.
A single driver can be connected to multiple receivers using a multidrop bus topology as shown below. LVDS is designed for point-to-point applications and so in a multidrop configuration, the number of receivers that can be connected and the signaling distance can be quite limited. MLVDS by contrast can be used in a multidrop topology to drive up to 32 nodes across longer distances compared to LVDS.
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 below. 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 (e.g. slave devices responding as broadcast commands are sent by the master to all nodes).
Another factor to be considered in multi-point buses is the bus idle condition. When no device is transmitting, the differential voltage on a terminated bus will be close to 0V. 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.