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Hello, welcome to our LVDS Educational Series with Texas Instruments. My name is Charlie Xie. In this video, I'll expand on those three communication topologies, provide an overview of M-LVDS, explain failsafe, and briefly go over B-LVDS. In a previous video, I have explained the LVDS standard and its operation.

I have established idea that LVDS is just a physical layer. M-LVDS is the same. M stands for multipoint. It is a physical layer standard first published in early 2002 as TIA NEIA 899. This standard brings all of the LVDS benefits to multipoint data transmission world. Before I talk about M-LVDS, let's look at the three different topologies.

The first one is point to point. A point-to-point interface consists of a single driver and a single receiver connected by transmission media. The point-to-point connection provides the optimal configuration from a signaling quality viewpoint, mainline stubs and other discontinuities avoided, and the highest possible signaling rate can be expected. The point-to-point topologies support simplex communication where the data transmission is unidirectional.

The second one is multidrop. In a multidrop system, a single driver is again used. But now multiple receivers are connected to the main transmission line. Where line termination is necessary, a single termination is located at the far end of the line from the transmitter. Like point-to-point, a multidrop connection provides unidirectional transmission.

The third one is multipoint. In a multipoint configuration, many transmitters and many receivers can be interconnected on a single transmission line. The key difference here is the presence of two or more drivers. Such a situation creates contention issues that need not to be addressed with point-to-point or multidrop system.

To support the location of the various drivers throughout the transmission line, double termination of the transmission lines is not necessary. Multipoint operation allows for bidirectional, half-duplex communication over a single balanced media pair.

Half-duplex is a term referring to a non-simultaneous bidirectional communication. Think of it as using walkie-talkie to communicate. You can talk both ways, but only one person can talk at a given time. In comparison, full-duplex is the term for simultaneous bidirectional communication, which allows both ends to communicate simultaneously.

With the understanding of these three topologies, let's look at why M-LVDS standard is needed. LVDS technology initially can only support point-to-point applications. And later, the standard was revised to support multidrop application. However, a multipoint application is still not supported by LVDS standard.

Multipoint application introduces a new set of problems. For example, the main transmission line need to be double terminated in multipoint system. Whereas, LVDS drivers specified for driving a 100 ohm load, a multipoint system using a 100 ohm transmission media, the load appears as a 50 ohm or even lower. With typical LVDS drive strengths, 350 milliwatt swing cannot be guaranteed.

Instead of overhauling existing standard, a new standard, M-LVDS was established, which included many unique features to address issues when operating with multipoint applications. There or a few key differences between LVDS and M-LVDS, which are presented in this table. The main difference is that M-LVDS has increased drive stress of 11.3 milliamp and 50 ohm drive load.

Why a multipoint system can be expected to look like 50 ohm to a driver. Closely spaced nodes can easily resolve in capacity of loading that drops the effective load seen by the driver to 30 ohm. The 11.3 milliamp output ensures that M-LVDS drivers still provide greater than a 300 millivolts under these conditions. M-LVDS also has a wider common mode input voltage to accommodate a possible range of voltages that can be presented to the bus.

A tighter sensitivity range was also introduced which provides improved noise margins when using M-LVDS devices. The only drawback with M-LVDS is the lower maximum data rates. This is needed to support [INAUDIBLE] multipoint system. The 1 nanosecond minimum transition time is specified which limited M-LVDS to 500 megabits per second, but the steps can be as long as 2 inches.

An added feature to M-LVDS standard is receiver failsafe. The LVDS standard does not specify any requirements for receiver failsafe. Receivers were simply specified to correctly detect the input state, where at least 100 millivolts was available. The M-LVDS standard has specified the two classes of receivers to bring standardization to failsafe implementation.

The standard uses the nomenclature, Type 1 and Type 2, to refer to these two types of receiver classes. Type 1 receivers are similar to LVDS receivers with improved threshold. Type 2 receivers has an offset threshold. Bus input signals that are less than 50 millivolts are defined to be low state. Greater than 150 millivolts result in a high state.

Type 1 receivers are expected to be used for maximum speed signals, such as data or clock lines. Type 2 receivers are useful for low-speed applications, such as control lines. Besides the LVDS and M-LVDS, B-LVDS is a commonly used signaling message. B-VLDS stands for bus LVDS.

Unlike LVDS and M-LVDS, bus LVDS is not standardized. It was invented by National Semiconductor, now part of Texas Instruments. It is similar to the implementation of M-LVDS, but with different drive strengths. B-LVDS should be considering heavily loaded background applications, where card loading and the spacing lower the impedance of the transmission line to as much as 50%.

TI offers a wide range of M-LVDS driver and the receiver that's suitable for different applications. For full portfolio, please go visit ti.com/lvds. Thank you for watching this video.

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