SSI: Compatibility of controller and SSI encoder

Topic:

• What maxon controllers can process SSI absolute encoder data?
• What specific details have to be taken into account concerning the compatibility of a SSI encoder protocol and the controller.

Situation:

SSI is an industry-proven and well established protocol standard in use by absolute encoders and linear scales since years. SSI single- and multi-turn encoders use a serial protocol to transmit the absolute position data to the motor controller.

At the early days of the SSI standard there was just position data transmitted by most SSI encoders. Many manufacturers of SSI encoders implement enhanced SSI protocols nowadays which transmit some additional status information of the encoder in addition. In principle this is helpful to get some information if the encoder had been referenced during initial commissioning or if there is an error state present (e.g. low battery in case of a battery back-up multi-turn encoder). This still complies with the SSI serial communication but the number of additional bits, the meaning of the bits, and the alignment of these bits in front or behind the position data can vary by each encoder manufacturer. Not all of these manufacturer specific SSI protocols and features might be compatible with any controller. It is even possible that the controller cannot properly process the position information mixed with status bits anymore.

In general most SSI encoders work with controllers supporting the SSI standard without any problems or special investigation based on a correct configuration of the controller.

If the SSI encoder does not just transmit position data but also additional information, there can be a compatibility issue sometimes. It is strongly recommended to investigate in the data sheet and details about the bit stream of the SSI encoder and compare this specification with the configuration features of the motor controller if there is a SSI encoder with more than 30 bit position data and / or additional "special" bits in use.

Solution:

EPOS4: Compatible with almost any SSI encoder protocol variant

The EPOS4 offers the most flexible configuration features for different SSI encoder protocol variants!

• The EPOS4 with firmware version 0x160(!!) or higher fits to almost any SSI encoder protocol variant. The very flexible protocol configuration allows to ...
  • ... process SSI data streams with up to 62 bits totally.
  • ... gather up to 16 bits of leading and / or trailing add-on data (beside the position).
  • ... to read even more than 32 bit of position data although just 32 position data bits will be processed for control. The position raw value can be still accessed by a master.
    
    
• Restrictions:
  Due to the fact that position control is based on 32 bit position data only any position data with more than 32 bit is tailored to 32 bit to be processed by the control loops. It is possible to configure what bits of the position's data raw value have to be ignored. Depending on the application there might be no need of full range of multi-turn bits and some of the most significant bits can be ignored. It might also be possible to ignore some of the lowest significant bits of the single-turn position data if te full resolution per one encoder shaft turn is not required by the application accuracy demands.
  
  
• Documentation:
  • The chapter "Setup / Connections / Sensor (X6)" of the EPOS4's product type specific "Hardware Reference" holds the wiring information of the SSI encoders.
  • Detailed information about the SSI encoder data configuration and processing can be found in chapter "SSI absolute encoder" of the "EPOS4 Firmware Specification".
    
    
• Configuration:
  The EPOS Studio "Startup" wizard makes it easy to adjust the EPOS4 configuration to the SSI protocol in use by an encoder. The graphical representation of the resulting bit assignment and processing helps to check the details, e.g. if the position data has more than 32 bits but just part of it will be processed by control.
  

MAXPOS: Compatible with lots of SSI encoder types

Lots of SSI protocol types are compatible with the MAXPOS but there are possibly some restrictions concerning the SSI encoder protocol selection have to be taken into account.

• Restrictions:
  The MAXPOS can process SSI data up to a length up to 30 bits totally (i.e. position data including optional trailing special bits).
  • If the transmitted data size is longer than 30 bits, the last part of the SSI data will be lost and the complete data stream (or position data) might not be processed properly at all.
  • In case of encoders with more than 30 bit position data it has to be checked by the encoder specification if part of the position data can still be processed properly and which restrictions might result (e.g. concerning encoder overflow and actual resolution / accuracy).
  • Any SSI encoder protocol variant with leading bits in front of the position data is cannot(!) be used by the MAXPOS and results in wrong position data processing.
    
    
• Documentation:
  • The chapter "Setup / Connections / 3.4.6 Sensor (X6)" of the MAXPOS "Hardware Reference" holds the wiring information of the SSI encoders.
  • Detailed information about the SSI encoder data configuration and processing can be found in chapter "SSI Absolute Encoder" of the "MAXPOS Firmware Specification".

EPOS2: Just two EPOS2 product types offer a SSI interface

Most of the EPOS2 controllers are designed for incremental encoder feedback only!

• Restrictions:
  • Just the product types "EPOS2 50/5" (PN: 347717) and "EPOS2 70/10" (PN: 375711) offer a SSI encoder interface.
    
  • Just "classical" SSI encoders which transmit position data only (without any additional special bits) can be in used.
  • The maximum total position data length is 31 bit.
    
    
• Documentation:
  • The chapter "13.3.1 SSI Absolute Encoder" of the "EPOS2 Application Notes Collection" holds the wiring information of the SSI encoders in use by the mentioned two EPOS2 product types.
  • Detailed information about the SSI encoder data configuration and processing can be found in chapter "SSI Encoder Configuration" of the "EPOS2 Firmware Specification".

ESCON: No SSI encoder interface at all!

SSI encoders cannot(!!) be in use with the ESCON product line.

• Remark:
  The ESCON product line is a pure velocity or current controller only. There is no need or an advantage by absolute position data in use for these control modes. Due to this the ESCON prodcut line does not provide a SSI encoder interface at all and there can just be encoders with incremental signals in use instead.

Additional notes:

Typically SSI characteristics which have to be checked for compatibility in advance

Please check the following details by the SSI encoder's data sheet and compare these with the configuration features of your controller especially if an EPOS2 or MAXPOS with limited configuration features is in use.

a.) Number of position bits (i.e. maximum resolution)
The number of multi-turn and single-turn position data bits as well as the total number of bits are limited to a specified minimum and maximum number by the controller. These limitations can be found in the controller's "Firmware Specification". Please ensure that the encoder's multi-turn and/or single-turn resolution does not exceed these configuration limits (in case of an EPOS2 or MAXPOS). Otherwise the SSI encoder cannot be used by EPOS2 or MAXPOS controller. Just the EPOS4 (firmware 0x160 or higher) has a very flexible feature to configure which extract of a too long position data has to be in use for position control.

• EPOS4:
  The EPOS4 can process position data with a length of maximum 62 bits but just 32 bits will be taken into account by control. The extract of maximum 32 bits in use by control as part of a longer SSI position data can be configured in a very flexible way.
  
  
• MAXPOS:
  There are maximum 30 position data bits supported.
  
  
• EPOS2:
  There are maximum 32 position data bits supported.

b.) Position transfer with a fixed number of data bits
Normally a SSI encoder exactly transmits the number of bits corresponding to the encoder's position data resolution which has to be configured by the controller too. There are some manufacturer of SSI encoders which transmit the position data always with a fixed frame length (= fixed number of bits) and fill up the frame with '0' bits if the actual position data resolution is lower. One popular manufacturer using such a position data frame with a fixed length is Renishaw.

The SSI encoders by Renishaw always use a fixed position data frame of 26 bits for some SSI protocols although the actual position data resolution might be lower (e.g. just 23 bit). The "unused" position data bits are filled up with '0' at the end. This means that the controller must process the actual position data only and ignore the '0' bits at the end. The '0' bits have to be ignored or processed like special bits, i.e. the position data has to be configured based on the actual resolution (e.g. of 23 bits) and the additional three '0' bits have to be configured to be ignored or as special bits.

• EPOS4:
  The EPOS4's very flexible configuration features (present with firmware 0x160 or higher) can typically configured based on the demands of such specific SSI protocol variants.
  
  
• MAXPOS:
  The MAXPOS might be limited by such a specific SSI protocol demands. There is some investigation required in advance if the encoder can be used based on its concrete data sheet and resolution.
  
  
• EPOS2:
  The EPOS2 can just process position data without any configuration possibilities for add-on  bits. The EPOS2 will probably not work properly with such specific SSI protocols variants. Some investigation based on the encoder's data sheet is required in advance. Anyway it is recommended to focus on the usage of the EPOS4 (instead of the EPOS2).
  

c.) Alignment and number of special bits
Some SSI encoders provide "special" bits in addition to the position data. 
It is possible to configure the number of special bits by the EPOS4 and MAXPOS within some limits. The bits can be read in a controller object by the master. The master can analyze and react on the special bit information then. Anyway there are some restrictions too:

• EPOS4:
  
  • There can be up to 16 leading and / or trailing special bits in use as long as the maximum SSI data size length of 62 bits is not exceeded.
    
    
• MAXPOS:
  
  
  • The number of trailing "Special bits" can be 8 in maximum.
    Remark:
    If a SSI encoder is based on fixed position data length (e.g. 26 bit) but not all of these actually correspond to the actual position resolution (e.g. 23 bit), the difference (e.g. 5 bits) count as "special" bits too if the frame was filled up with '0' bits at the end.
  • Position data has to be transmitted first by the SSI protocol, i.e. "Special bits" have to follow the position data finally.
    If there is a SSI protocol present which transmits the "Special Bits" in front of the position data, the SSI encoder is not(!) compatible with the MAXPOS. 
    
    
• EPOS2:
  The EPOS2 cannot(!) process "Special Bits". The position data might be possibly wrong if there are "Special Bits" present by the SSI encoder's data stream.

d.) Clock frequency & timing

Normally the configuration of the clock frequency and timing does not provide any difficulties due to often wide range of supported clock frequencies by the SSI encoder and wide range of configurable settings by the controller. There will be just a quick look in the SSI encoder's data sheet required to configure the controller properly.

e.) Power consumption

Typically the 5V output current of 100 mA (or even 150 mA depending on the concrete controller) is sufficient to supply an encoder. There are just some encoders (e.g. Renishaw) which partly require a higher supply current due to additional features or a powerful internal processor. Please check the power consumption of the encoder and compare it to the 5V output current rating provided by the controller. If the current draw by the encoder would be too high, there is the need for an additional 5V supply. The GND of the external 5V supply and the controller's logic GND has to be connected then.

Further assistance required?

maxon cannot take responsibility that any SSI encoder type is compatible with maxon controller. There is just a too great variety of different and enhanced SSI protocol types present by the different encoder manufacturers. If you are not sure about the compatibility of a SSI encoder and maxon controller, please "Submit a request" by the Support Center and provide the following information:

• Manufacturer name and full part number of the encoder.
• Data sheet or manual of the encoder holding the technical data and protocol specification.
• Part number or product name of the maxon controller.
• *.dcf configuration file if there is an EPOS2 or EPOS4 in use but does not work like expected.

Related document links:

• EPOS4: SSI configuration features
• MAXPOS / BISS-C: Compatibility of BISS encoders