In the context of CAN (Controller Area Network), RTR stands for Remote Transmission Request.
The Remote Transmission Request (RTR) is a specific bit within a CAN data frame that is used by a node to request data from another node on the network.
Understanding the RTR Bit
According to the provided reference, the RTR–The single remote transmission request (RTR) bit is dominant when information is required from another node.
Here's a breakdown of how it functions:
- Purpose: Instead of sending data, a node wanting information from another node sends a Remote Frame. This frame contains the identifier (ID) of the data it is requesting.
- The Bit: The key difference between a standard Data Frame and a Remote Frame is the state of the RTR bit. For a Data Frame, the RTR bit is recessive. For a Remote Frame (a request), the RTR bit is dominant.
- Requesting Data: When a node needs specific data (identified by its standard or extended identifier) from another node, it transmits a Remote Frame with the RTR bit set to dominant.
- Targeting the Node: All nodes receive the request, but the identifier determines the specified node. Only the node that transmits the data associated with that specific identifier is expected to respond.
- Responding: The targeted node, upon receiving the Remote Frame and recognizing its identifier, should then respond by transmitting a standard Data Frame containing the requested data.
- Data Distribution: The responding data is also received by all nodes and used by any node interested. This highlights the broadcast nature of CAN communication; even though one node requests the data, all nodes on the bus can listen to and use the response if they are configured to do so.
RTR Bit Functionality Summary
| Frame Type | RTR Bit State | Purpose | Data Field |
|---|---|---|---|
| Data Frame | Recessive | Transmitting Data | Contains data |
| Remote Frame | Dominant | Requesting Data | Empty |
Why Use RTR?
Remote Frames and the RTR bit offer a way for nodes to initiate communication and request specific information rather than relying solely on other nodes to periodically broadcast data. This can be useful in scenarios where data is only needed on demand, potentially saving bus bandwidth compared to continuous broadcasting.
Practical Insights:
- RTR frames are less commonly used in modern CAN implementations compared to direct data transmission, partly due to potential bus load issues if many nodes request data simultaneously.
- Nodes must be specifically programmed to recognize Remote Frames addressed to their identifiers and respond accordingly with a Data Frame.
- A Remote Frame contains the identifier and the Data Length Code (DLC), specifying the amount of data being requested, but the data field itself is empty.
In essence, the RTR bit signals whether a CAN frame is carrying data (Data Frame) or asking for data (Remote Frame).
