Open vs. Closed Encoder Communication Protocols:
How to Choose the Right Protocol For Your Application
Mr. Cory Mahn
Applications Engineer
Danaher Industrial Controls Group
Gurnee, IL
I. Introduction
Imagine that you are specifying an encoder to provide motion control feedback. You have
already decided whether the encoder needs to be incremental or absolute, and you now have to
decide on the communication protocol. The application has too many devices to use parallel
communication, and too few to require a bus such as DeviceNet or Profibus. The best choice
appears to be a serial communication link. Unfortunately, the numerous choices in serial
communication links can make the choice a confusing task. On what basis should you choose
one protocol over another? This application example highlights the key questions typical for
most applications. This paper is intended to answer and demystify some of these questions and
help you choose the right serial communications protocol for your encoder application.
II. Open vs. Closed Communications Protocols:
What does it mean to me and my application?
You might have noticed that some communication protocols are available from a limited
number of manufacturers. Chances are that these protocols are closed-type communications
protocols. Closed communication protocols are proprietary, meaning that they require a user
license and license fee for users to design their own interface with a closed protocol slave.
Detailed specifications on a proprietary protocol are restricted; they are available to license
holders and not the general public. Closed-protocol devices are essentially ‘black-box’ devices
because not much is known about the specifics of their operation. The most common closed
serial communication protocols for encoders include EnDat® and HIPERFACE®, which were
developed by individual encoder manufacturers.
Open communication protocols, on the other hand, are non-proprietary and non-restrictive.
The developers of communication protocols freely share specifications regarding data format
and electrical design. Designers of products that use serial communications, such as sensors
and encoders, can create their own interfaces without paying a licensing fee. This means that –
when you are shopping for a replacement serial communication device – you’ll find many more
encoders available on the market. The most popular open communications protocol is
Synchronous Serial Interface (SSI). SSI is a well accepted and time-tested protocol. More
recently, the Bi-directional Synchronous Serial Interface (BiSS) open serial protocol has come
into the market. Using SSI as a basis, it represents an alternative to bi-directional
communication protocols such as EnDat® and HIPERFACE®.
III. Why use a closed communications protocol?
There is one good reason to use a closed proprietary protocol; simplicity. For instance, the
communication protocols slave configuration software is usually pre-designed, saving time and
development costs. Another reason may be that a user prefers a particular vendor’s product
and agrees to pay the licensing fee in exchange for the ability to completely rely on that
manufacturer for all levels of required support for the encoder or sensor. Of course the
drawback to this approach is that the user limits his/her options in the future, when a
replacement device is needed.
IV. Why use an open communications protocol?
The key benefits to using an open communication protocol include:
• Availability – Encoder buyers have more selection options and alternative
manufacturers to choose from.
• Cost –Manufacturers of products using closed communication protocols can charge
any fee that the market will bear for a replacement encoder, since there is virtually no
competition from other manufacturers. On the other hand, competition among suppliers
of open-communication-based products promotes price and product alternatives.
• Information - Closed protocol developers limit the amount of information that is
published and available to buyers. However, if the need exists, an educated user can
check the compliance of an open protocol from a specific manufacturer. Information
regarding that protocol is freely posted on the internet.
V. How do the various communication protocols operate?
In order to select the right protocol, buyers must fully understand the operating capabilities and
functional limitations of each of the available feedback control protocols.
SSI
SSI is the foundation on which all of the aforementioned protocols are based. SSI
communication requires two twisted pair wires plus two wires for power. Encoders may
also be supplied with additional incremental outputs for speed feedback. For serial
communications, one pair of wires is for a differential clock signal and the other pair is
for data feedback from the sensor. Clock frequencies can be as high as 1.5MHz.
However, as clock frequency increases, the maximum cable run decreases, which is a
limitation common to all protocols.
Cable Length Data Rate
50 m 400 KHz
100 m 300 KHz
200 m 200 KHz
400 m 100 KHz
Table #1, Cable Length vs. Data Rate
The data frame length depends on the device and its resolution. In an SSI interface,
there is one slave per master, referred to as a point-to-point connection. The clock
remains high until the master needs information from the sensor. It then sends a stream
of clock pulses equivalent to the number of bits of information from the sensor.
Samples of each bit are usually taken on the falling edge of the clock pulse. This
insures that the propagation and process delays are accounted for. Sensor
manufacturers are free to insert alarm bits if required, but the requirement must be
provided before the product is shipped, and a single alarm bit may have several
meanings. See Figure 1 for an example of what an SSI trace may look like.
Figure #1, SSI Communication Format
VI. EnDat®
EnDat is a proprietary protocol created by Heidenhain of Germany. EnDat is similar to
SSI in that it is a point-to-point connection. EnDat is also a synchronous protocol like
SSI, meaning that the data, with a speed of 4 MHz, is received as clock pulses are sent
simultaneously. However, much more data can be read from an encoder and stored
due to the provided internal memory. This can include diagnostics, identification,
alarms, and warnings. It can also contain information about the motor in which it was
mounted to such as model and serial numbers. Varying clock pulse stream lengths are
also required from the varying data that may be received or sent to the device. Also,
using a function called a Datum Shift, an encoder can easily be reset to a new zero or
reference point. The Datum Shift is a value that is added to the physical position of an
encoder. SSI provides a direct reading of the physical position, so it requires the end
user to rotate the shaft to the zero position.
The hardware level minimally requires six wires for communications. Depending on the
version of EnDat, there may be an additional wire that carries an analog incremental
output for faster speed sampling. This is useful for speed-controlling drives that can be
more demanding than the rate at which serial feedback is normally provided.
For more information on the EnDat protocol refer to the Heidenhain website at
www.heidenhain.com.
VII. HIPERFACE®
HIPERFACE is a proprietary serial protocol created by Max Stegmann GmbH.
HIPERFACE can be point-to-point like SSI and EnDat, but it can also use bus
connections in which several encoders can be wired together and addressed by a
single master. HIPERFACE is the only one of the four major serial protocols that is
asynchronous. It uses bidirectional RS-485 communications to send and receive data.
The data comes at a relatively slower rate of 38.4Kbps, but contains much more
information than provided in SSI communications.
Like EnDat, it has internal memory that can hold motor information such as voltage,
current, and other parameters. Using a licensed master, you can also define an
absolute zero position. Because HIPERFACE is proprietary, the user doesn’t know how
the communications work and how parameters are addressed. Setting and storing
parameters are essentially “plug and chug” with the manufacturer provided software.
Generally, the data sent and received from the master is at a fixed length and is not
controllable by the end user or OEM.
At the hardware level, eight wires are required. The proprietary SinCos® output requires
four wires for each differential pair, with two wires required for data communication and
two for power.
For more information on HIPERFACE, refer to the Stegmann website at www.sickstegmann.
de.
VIII. BiSS
BiSS is the latest protocol designed and developed by IC-Haus, Germany. It was
created to be an open non-proprietary protocol alternative to EnDat® and HIPERFACE®
but designed with the capabilities of the proprietary protocols in mind. Combined
features such as alarms, warnings, and the ability to store motor information to the
encoder exist in the BiSS protocol.
BiSS has two modes; sensor mode and register mode. In sensor mode, the sensor or
encoder communicates in a manner similar to SSI. The master begins to send a stream
of clock pulses. Eventually the data line level will drop low and data sampling will begin.
The data can be received and clocked at a blazing 10MHz. Due to the speed of
transmission, many drives may not require additional analog incremental outputs to
control motor speed. In ACURO encoders available from Danaher Industrial Controls
Group, the propagation and calculation delay is so brief that communication is
backwards compatible with the SSI protocol. Figure 2 shows the communication format
and a data frame map.
Figure #2, BiSS Sensor Mode
In register mode, the protocol modulates the clock pulse width to address specific
slaves and parameters. This mode is unlike any other protocol. If in the sensor
mode a warning or alarm bit is set high by the sensor or encoder, the end-user may
access the register mode and find specifics on the alarm or warning. This might be
an over-temperature warning or, in the case of an encoder, a disk pollution alarm.
Danaher Industrial Controls Group is currently developing the ability to provide
single step alarms in case improper position is being provided in sequence. Other
information such as acceleration, temperature, and identification can be retrieved
from an encoder with BiSS. Also, as mentioned before, register mode allows for
sending and storing data to the encoder. Figure 3 and 4 illustrate the
communication format and data map of a BiSS device in register mode.
Figure #2, BiSS Register Read Mode
Figure #3 BiSS Register Write Mode
On a hardware level, the same cable used in SSI and EnDat can be used with BiSS
applications. For more information on IC-Haus and the BiSS protocol , refer to the
BiSS Website at
www.biss-interface.com.
IX. Summary of the Four Dedicated Serial protocols
Table 2 shows some of the key differentiators for each protocol.
Table #2, Serial Interface comparison
X. Conclusion
Choosing the right communication protocol for an encoder application can be difficult. This
primer gives OEM project engineers and end-users enough knowledge of the basic
operating principles to make an educated buying decision. Encoders that offer open
communications protocols offer the most flexibility, and new non-proprietary protocols such
as BiSS will accelerate the trend away from using closed communication protocols.
