Tmds Data 2+

[Candi Ruman]

Transitionminimized differential signaling (TMDS) is a technology for transmitting high-speed serial data used by the DVI[1] and HDMI video interfaces, as well as by other digital communication interfaces.

The transmitter incorporates an advanced coding algorithm which reduces electromagnetic interference over copper cables and enables robust clock recovery at the receiver to achieve high skew tolerance for driving longer cables as well as shorter low-cost cables.

The method is a form of 8b/10b encoding but using a code-set that differs from the original IBM form. A two-stage process converts an input of 8 bits into a 10 bit code with particular desirable properties. In the first stage, the first bit is untransformed and each subsequent bit is either XOR or XNOR transformed against the previous bit. The encoder chooses between XOR and XNOR by determining which will result in the fewest transitions; the ninth bit encodes which operation was used. In the second stage, the first eight bits are optionally inverted to even out the balance of ones and zeros and therefore the sustained average DC level; the tenth bit encodes whether this inversion took place.

The 10-bit TMDS symbol can represent either an 8-bit data value during normal data transmission, or 2 bits of control signals during screen blanking. Of the 1,024 possible combinations of the 10 transmitted bits:

Control data is encoded using the values in the table below. Control data characters are designed to have a large number (7) of transitions to help the receiver synchronize its clock with the transmitter clock.

On Channel 0 the C0 and C1 bits encode the Horizontal synchronization (HSync) and Vertical synchronization (VSync) signals. On the other channels they encode the CTL0 through CTL3 signals which are unused by DVI but in the case of HDMI are used as a preamble indicating the type of data about to be transferred (Video Data or Data Island), the HDCP status and so on.

TMDS is similar to low-voltage differential signaling (LVDS) in that it uses differential signaling to reduce electromagnetic interference (EMI) which allows faster signal transfers with increased accuracy. TMDS also uses a twisted pair for noise reduction, rather than coaxial cable that is conventional for carrying video signals. Like LVDS, the data is transmitted serially over the data link. When transmitting video data and used in HDMI, three TMDS twisted pairs are used to transfer video data. Each of the three links corresponds to a different RGB component.

The physical layer for TMDS is current mode logic (CML),[2] DC coupled and terminated to 3.3 Volts. While the data is DC balanced (by the encoding algorithm), DC coupling is part of the specification. TMDS can be switched or repeated by any method applicable to CML signals. However, if DC coupling to the transmitter is not preserved, some transmitters' "monitor detection" features may not work properly.

Several roadblocks can impede the optimal exchange of technical information. The most notorious is the improper capture of information at the time of test or simulation. All too often data is stored without descriptive information, in inconsistent formats, and scattered about on arrays of computers, creating a graveyard of information that makes it extremely difficult to locate a particular data set and derive decisions from it. When data sets cannot be located, tests or simulations must be recreated. As a result, many companies see decreased efficiency and drastically increased development costs. To meet these challenges, NI has defined a technical data management (TDM) solution that includes three integral components:

You can choose from a variety of format options for measurement data storage. Unfortunately, careful consideration of data storage options is not typically at the forefront of application planning. The file format choice is often overlooked in favor of higher-visibility decisions such as hardware system design or software architecture. Data storage decisions are sometimes made arbitrarily or on an as-needed, per-application basis without second thought for reusability and scalability, which leads to complex and costly software rearchitecture. Because applications and requirements change over time, even the most popular traditional storage formats quickly fall short of meeting the demands of engineers and scientists storing time-based measurement data. Table 1 shows the pros and cons of some of the most commonly chosen storage options for measurement data.

NI introduced the Technical Data Management Streaming (TDMS) file format as a result of the deficiencies of other data storage options commonly used in test and measurement applications. The binary TDMS file format is an easily exchangeable, inherently structured, high-speed-streaming-capable file format that, when combined with the other technologies in the NI TDM solution, becomes quickly searchable without the need for complicated and expensive database design, architecture, or maintenance.

At each level of the hierarchy, you can store an unlimited number of custom scalar properties. Each level accepts an unlimited number of custom-defined attributes to achieve well-documented and search-ready data files. The descriptive information located in the file, a key benefit of this model, provides an easy way to document the data without having to design your own header structure. As your documentation requirements increase, you do not have to redesign your application; you simply extend the model to meet your specific needs. The more custom properties you use to document your measurement data, the more easily it can be located at a later date by using an NI DataFinder client that abstracts complex database communication from the user.

1TDMS files also automatically generate a complimentary *.tdms_index file. This file provides consolidated information on all the attributes and pointers in the bulk data file that drastically speeds up read access to the data on larger data sets. This index file is not required for storage or distribution and automatically regenerates.

The NI TDMS file format is an NI platform-supported file format. All NI software development environments interface with TDMS files as part of their native function palettes or libraries. These interfaces abstract the complexity of storing structured data while making it easy to add descriptive information along with captured measurement or simulation data.

There are multiple interfaces to NI TDMS files from NI LabVIEW software. The easiest way to get started writing TDMS files in LabVIEW is with the Write to Measurement File Express VI. This Express VI offers the ease of dialog-based configuration but sacrifices performance and is not suitable for high-speed streaming or real-time applications.

For more flexibility and to achieve the best performance, use the TDMS primitive VIs from the File I/O palette. With these VIs, you can read and write TDMS files and their properties in the most efficient manner possible. This method of accessing TDMS files is real-time-capable using the LabVIEW Real-Time Module. The TDM Streaming palette was introduced in LabVIEW 8.2.

Installation of LabVIEW or drivers released in August 2010 or later includes access to a brand new TDMS Advanced palette for extremely low-level control of TDMS files, so you can perform advanced techniques such as asynchronous writes and reads.

Note: Support for the DIAdem Connectivity Library was removed in LabWindows/CVI 2017. NI recommends that you use the TDM Streaming Library for existing projects and the TDM C DLL (attached) for new projects.

NI offers the TDM C DLL as a free download. It contains the necessary functions for reading and writing TDMS files from any application development environment that is flexible enough to enable DLL communication. To download the DLL and corresponding examples for free, download the TDM C DLL from the attachments section of this page.

With the free add-in for OpenOffice.org Calc, you can load and process TDMS files including descriptive information in OpenOffice.org Calc. Just download the add-in, install it, and use the OpenOffice.org Calc functions with an additional menu to load TDMS files and to configure the add-in.

Sometimes it is impossible to use the TDMS file format. For example, occasionally customer or supplier requirements dictate that you must use a particular file format for data storage. Certain traditional instruments automatically provide data output files using a custom format. Furthermore, legacy measurement data that has already been collected in a particular fashion cannot be recollected simply to store it in the TDMS file format.

NI offers hundreds of free DataPlugins for you to download. To download a DataPlugin for the most common file formats to use the TDMS model, or to request a DataPlugin be written for free for your custom file format, see ni.com/dataplugins.

With NI-DAQmx 9.0 and later, you can log data to TDMS files from directly within the DAQmx API. By configuring logging via the DAQmx Configure Logging VI, you can easily integrate TDMS logging into existing applications. Furthermore, this method of streaming data to disk helps you truly push the boundaries of high-speed measurement data streaming by optimizing several memory operations and bypassing Windows, LabVIEW, and TDMS buffers for maximum efficiency. Tests with the DAQmx Configure Logging VI have realized data streaming rates of more than 1.2 GB/s. To learn more about how to integrate TDMS logging within your NI-DAQmx application, see NI-DAQmx High-Speed Streaming to Disk.

NI developed the TDMS file format to help engineers and scientists properly store the large amounts of data they generate during simulation and test. With an easy-to-use interface for storing well-organized and documented files, you can focus your efforts on more pressing areas of your applications and let the data storage aspect of your application interface scale naturally with your application.

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