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Q1 : What is DSI?

A: DSI is a Distributed System Interface intended for powering and communicating with remotely located slave devices in a self-contained automotive network operating over a 2-wire twisted pair. The slave devices are typically acceleration sensors responding with raw-g data or pressure sensors responding with relative pressure data.

Current production slave devices are implemented as state machines but can contain any level of complexity required for the intended application.

Q2 : What are the features of DSI compared to other sensor interfaces (such as CAN, LIN, Flex Ray and PSI5)?

A: * LIN bus is a low speed (20kbps) single wire single master multiple slave network SCI communication network. All transactions are initiated by the master. Additional nodes can be added or removed from the bus without physical changes to the node hardware.

* CAN bus is a medium speed (125kbps on single wire to 500kbps on a 2-wire balanced line) protocol. The CAN protocol provides bus arbitration at the physical layer through recessive and dominate bit-states on a bit by bit basis with lower numbered address having higher priority. Quartz-based clocks are required at all nodes (CSMA / CSMA-CD). Carrier sense multiple access with collision detection

* FlexRay is a high speed (2.5 to 10Mbps), dual channel, multi-master distributed system architecture which offers deterministic communication to support safety-critical applications. FlexRay facilitates reduced wiring compared to parallel CAN networks when employed as the vehicle wide network backbone. Highly complex control modules and physical layers required at each node.

The LIN, CAN and FlexRay bus all require self powered nodes with a common ground reference. Slave nodes typically require 3 or 4 wires, 2 for power and 1 or 2 for signal. In the case of single wire CAN and LIN the reference wire for communication can be the power return.

* PSI5 is a medium speed (125kbps to 189kbps) 2-wire bus-powered master/slave Manchester encoded communication network that operates in synchronous or asynchronous modes. Synchronous systems support a multiple access TDMA concept that allows connection to up to 3 (at 125kbit/s) or 4 (at 189kbits/s) sensors. The standard supports half-duplex bidirectional communication.

* DSI bus is a medium speed (125kbps to 200kbps) 2-wire bus-powered master/slave full duplex communication network. The bus operates a synchronous mode with an efficient full duplex bi-directional communication with up to 15 slave sensors or actuators. The protocol specifies the physical layer and only the essential elements of the data link layer, allowing for greater flexibility. Physical layer can be either single ended or differential for enhanced noise performance.

Q3 : How can I get the DSI standard?

A: The current DSI standard is available for free download from this site by selecting "Downloads" from the menu block. In order to access downloads, we ask for your name, company and email address. Optionally, you can also choose a username and password for future convenience.

Q4 : What is the target scope of DSI and what are possible applications?

A: The initial application for DSI is in the automotive safety market, specifically airbags. Over 200 million DSI devices are now in production vehicles. There are other potential automotive applications including powertrain, vehicle dynamics, and ambient lighting, as well as industrial uses such as distributed control and garage door systems.

Q5 : Which DSI components are available in the market?

A: Freescale offers a number of devices that implement the DSI standard, including the MC33780 2-Channel Bus Master, the MC33781 4-Channel Bus Master, and the MC33793 and MC33784 Slave devices.

In addition, a wide variety of pressure and inertial sensors with integrated slave DSI interfaces is available. Click here to view available products.

Q6 : What kind of connectivity configuration is possible? (e.g. Point to Point, Parallel, Loop and Daisy Chain...)

A: DSI standard supports point to point, parallel and daisy-chain configurations. For point to point and daisy chain configurations, the slave device addresses can be specified during initialization sequence or the addresses could be pre-programmed. For parallel configuration the addresses must be pre-programmed.

In cases where it is necessary to work around bad sections of the wiring harness in a multi-node system, the loop configuration can be employed. The loop configuration allows the system to be configured as a loop whereby the nodes can be initialized from either the forward or the reverse order, or if a harness error exists a combination of forward and reverse initialization can avoid harness sections with open or short circuits.

Current production devices do not support loop configuration harnesses.

Q7 : What is the the maximum number of nodes? What are the limiting factors e.g. power supply voltage, communication speed, etc.

A: There are practical limits to the number of nodes that can be addressed in a system. The primary limiting factor is available bus bandwidth. For example, for any given bit rate and sample rate there is a fixed amount of time available for that is based on the message length and the inter-frame separation.

A secondary constraint is that the DSI message only allocates four bits to the slave address. Since slave address 0000 is used for global commands, there is a theoretical upper limit of 15 devices per channel (0001 through 1111).

Q8 : How can I calculate the delay time of the communication?

A: The communication delay time through the signal chain is discussed in detail in Section 5, "Data Flow Through The DSI System", of Freescale Application Note AN3670, "Implementing a DSI Network Using the MC33781 and the MC33784". Click here to view.

Q9 : What is the purpose of having either single-ended or differential communication?

A: The DSI standard allows for either single-ended or differential bus communication configurations. With a single-ended bus communication the entire 3-state voltage waveform is produced on a single wire with the other wire serving a voltage reference and power return. With the differential bus communication the master produces a portion of the DSI voltage waveform on each of two wires such that the voltage difference between the positive and negative Bus wires are as defined by the DSI standard. Typically the positive and negative wire produce ½ of the waveform on each wire; however, the bus standard only requires that the voltage difference between the positive and negative bus wires conform to the standard. The slave device cannot determine if the bus is single ended or differential.

Controlling only the voltage difference between the positive and negative wires allows varying levels of sophistication while sill satisfying the standard. As a result there is flexibility within the standard to differentiate a particular implementation over competing implementations.

Q10: How can I make sure that our product will conform to the DSI standard?

A: Conformance to the DSI standards is expected to consist of a self-certification exercise coupled with the agreement from the OEMs that introduce devices into series production. The consortium plans to solicit participation of automotive test and assurance providers to create and execute evaluations of solutions using the standard as reference. Evaluations are expected to be of a "pass/fail" nature to encourage innovation in implementations towards cost reductions &/or robustness improvements.

Q11: What kind of evaluation tools are available?

A: There is limited availability of third party tools available for the DSI bus. A goal of the DSI Consortium is to address this need for third party tools. Alternatively, being a Consortium member allows members to freely exchange tools that are developed by Consortium members. For example TRW and Freescale have exchanged some tools as a matter of course while developing new products for the DSI bus.

Freescale can provide interested parties with evaluation tools for all of the products that it produces

Q12: Is sample code available to assist in software development?

A: The consortium, depending on availability of resources, may develop initial reference designs that could be used as starting points for participant development activities. The consortium envisions that continual innovation will lead to the evolution of specific implementations over time.

Q13: Is there a charge to use the DSI Specification? Do I have to pay royalties?

A: The consortium envisions automotive applications to be free of royalties.

The DSI standard will not be precluded from non-automotive uses, however reasonable and customary royalties may be required for non-automotive applications.

Q14: How will I know when the specification is updated?

A: Adopters will gain from the earliest developmental, drafts, and work-in-progress information as part of participation in the various work groups. As standard revisions become approved, new postings will appear on the website and highlighted in the What’s New section.

Q15: How can I become a member of the DSI consortium?

A: Contact one of the persons listed in the Contacts section of the website. Then review the Adopter Agreement, and complete the application for approval. Once approval is granted, a solicitation to begin participation in the various work groups will soon follow.

Q16: What is the benefit of becoming a member of the consortium?

A: The primary benefit will be the ability to participate in the development of the next and follow-on versions of the standard, and therefore bring innovative implementations to market in the shortest timeframes.

Q17: Is there an admission charge or annual fee?

A: The consortium does not envision the need for fees. Adopters will be asked to provide human resources and their support for the various work groups, at a level commensurate with the participating organization expectations.