Foreground communication

Foreground communication is intended for data that must be available in the communication cycle (e.g., controller cycle).

• Requests and responses are structured in frames with a defined length. Each request and response begins with a preamble (PRE) and ends with a postamble (POST).
• The REQ request frame controls communication with the encoder or triggers certain actions within it (e.g. clearing of error messages) and therefore also determines the contents of the response.
• Regardless of the REQ frame contents, the response frame is chronologically divided into high-priority data and low-priority data. 

A response contains the following elements:

One HPF (High Priority Frame)
The HPF typically contains the encoder position. Depending on the encoder, other information can be specified for transmission in the HPF as well.

One LPH (Low Priority Header)
The LPH contains status information about the subsequent data contents. It also contains information about the send list and the number of transmitted LPFs. The send list specifies the chronological sequence of LPFs within the individual communication cycles.

Up to 15 optional LPFs (Low Priority Frames)
The LPFs transmit additional data, such as:

• Diagnostic values
• Sensor information
• Redundant information for functional safety

The LPFs used in EnDat 3 continue to build on the concept of additional data found in EnDat 2.2. The switch between various LPFs is performed in accordance with a send list configured in the encoder’s memory. Intervention in the controller cycle by the subsequent electronics is not required.

The send list can be configured either in the encoder’s volatile memory after each restart or permanently in its non-volatile memory. During operation, the send list specifies which LPFs are to be included in the response from cycle to cycle. Up to eight different send lists can be stored in the memory. The type of request determines which send list is active, thereby allowing the subsequent electronics to react to various operating statuses with speed and flexibility.

A sample communication cycle is shown below:

• A complete cycle always contains the white fields.
• Up to 15 optional LPFs (marked gray) can also be included.
• The REQ, HPF, LPH, and individual LPF protocol contents are each safeguarded by a CRC (Cyclic Redundancy Check).

Background communication

Some tasks, such as reading from and writing to the encoder memory, have low timing demands. For these kinds of tasks, EnDat 3 defines a background channel. Background communication is embedded in the foreground communication and uses its frames as transport carriers (REQ, LPH, LPF). The background channel thus makes it possible to read from and write to the encoder memory in the controller cycle. However, the background channel cannot handle real-time demands.

Bus operation

Along with point-to-point mode, EnDat 3 also offers bus operation for special applications. In bus operation, a Bus Request Frame is added in front of the Request Frame, thus allowing multiple participants to send responses in a single communication cycle.

Diagnostics

EnDat enables extensive encoder monitoring and diagnostics without an additional wire. The interface’s diagnostics are an essential precondition for high complete-system availability. They generate the following aids:

• Valuation numbers
• Error messages
• Warnings

Key factors:

• Machine utilization planning
• On-site support for the service technician
• Easy evaluation of the encoder’s function reserve
• Simplified troubleshooting for repairs
• Generation of useful quality statistics

For analysis of encoder functionality, valuation numbers can be read cyclically from the encoder.

Valuation numbers do the following:

• Provide information about the current status of the encoder
• Allow an encoder’s function reserve to be determined

Their identical scaling for all HEIDENHAIN encoders enables consistent analysis.

The function reserves, combined with other sensor data, serve as the basis for condition monitoring and predictive maintenance in the higher-level subsequent electronics.

EnDat master

The EnDat 3 master handles communication with EnDat 3 encoders from HEIDENHAIN. This provides a convenient way to transfer EnDat 3-transmitted information to the higher-level control. Due to the high transmission frequencies, an FPGA or ASIC is usually employed. For integration into an FPGA or ASIC, HEIDENHAIN provides an EnDat 3 master in the form of VHDL code.