When you specify an absolute encoder, the protocol choice is as important as resolution. It determines your controller hardware, your update rate, your safety overhead and — in many cases — which encoder suppliers you can even consider. Here is a practical comparison of the three protocols the RM series supports.
RS485 (Tamagawa protocol)
RS485 is the physical layer used by the Tamagawa absolute encoder protocol, which became the de facto standard for servo motor feedback — especially in Japanese and Chinese servo ecosystems. The drive sends a request and the encoder replies with absolute position over a differential RS485 link.
- Best for: servo motors and drives already designed for Tamagawa-compatible encoders; drop-in replacement programs.
- Advantages: enormous ecosystem compatibility, proven field reliability, simple point-to-point wiring.
- Limits: lower bandwidth than BISS-C; the protocol is proprietary to the Tamagawa ecosystem.
BISS-C
BISS-C is an open, high-speed bidirectional protocol. The controller clocks the encoder and receives position data, while a CRC ensures data integrity on every frame. It supports extremely fast position updates — ideal for high-dynamic axes.
- Best for: high-dynamic robot joints, high-performance servo axes, applications needing real-time safety checks.
- Advantages: open standard, high speed, CRC protection, broad chipset support in modern drive ASICs.
- Limits: slightly more complex controller-side implementation than SSI.
SSI
SSI (Synchronous Serial Interface) is the simplest industrial absolute-encoder interface: the controller clocks out position bits synchronously. It has been around for decades and is supported by virtually every PLC and motion controller.
- Best for: general industrial machines, PLC-based systems, applications where simplicity and cost dominate.
- Advantages: dead simple, low-cost, universally supported, robust.
- Limits: slower and one-directional; no advanced diagnostics.
Side-by-side
| Criterion | RS485 (Tamagawa) | BISS-C | SSI |
|---|---|---|---|
| Direction | Bidirectional | Bidirectional | Unidirectional (clock/data) |
| Data integrity | Protocol-dependent | CRC on every frame | None / parity optional |
| Speed | Medium | Highest | Low-medium |
| Open standard | No (ecosystem) | Yes | Yes |
| Servo ecosystem fit | Excellent | Excellent (new drives) | Good |
| Implementation effort | Medium | Medium-high | Low |
| Cost | Medium | Medium | Low |
How to choose
- Replacing an existing servo encoder? Match the protocol already in your drive — RS485 for Tamagawa systems.
- Designing a new high-performance servo or robot joint? Choose BISS-C for headroom, speed and CRC safety.
- Integrating into a PLC with minimal engineering? SSI is your fastest path.
- Mixed environment? Pick a supplier whose encoders support all three on one terminal — like the RM series — so one qualified product covers multiple machine lines.
Why protocol flexibility matters in supply
Locking your design into one proprietary encoder ecosystem reduces your negotiation power and exposes you to single-source risk. Specifying a supplier that can deliver the same core technology over multiple protocols — with the same pinout and the same factory support — keeps your options open without multiplying qualification work.