TB20 SSI encoder interface

In terms of hardware, the module’s SSI is implemented as a point-to-point connection as defined in the RS-422 standard. Moreover, in order to avoid ground loops, opto-isolators are used to achieve full electrical isolation.
Excellent noise immunity is ensured by the fact that the clock and data signals are sent through twisted pair cables using synchronous and symmetric transmission. The result is high reliability and robustness, making the module ideal for use in industrial environments.
The TB20 SSI encoder interface makes it possible to connect an SSI encoder with a transmission rate of up to 2 MHz. And, since the number of data bits is also adjustable, the module provides utmost flexibility in terms of supported encoder resolutions.
Data values can be in binary code or reflected binary code (Gray code), and can be normalized so as to ensure that only position-relevant bits will be transmitted.
In addition, a parity bit can be optionally evaluated in order to check data integrity. Also, special bits can be transmitted, which are used by some sensors to perform tasks such as signaling error states to the master.
A digital 24 V input can be configured as a latch function in order to retain the encoder value when specific external events occur.
Moreover, two configurable comparators make it easy to quickly detect positions in the PLC program.
In addition to the encoder value, all relevant encoder interface status information is available in the PLC input image table.

  • Accommodates an absolute encoder as per RS-422
  • Supports 13 to 31 data bits
  • Single-turn and multi-turn
  • Encoder value normalization (the encoder value will only contain position-relevant bits)
  • Direction indicator display
  • Direction reversal
  • Internal Gray-binary conversion
  • Supports transmission rates of up to 2 MHz
  • Encoder value parity check
  • Re-transmission
  • Digital 24 V input
  • "Latch function" for freezing the current encoder value
  • Comparator function with loadable reference values, configurable depending on the direction of rotation
  • 24 V encoder supply
1SI Features
Number of SSI decoders1
Supported data bits13 … 31 bits
ParityNone, even, odd
Internal Gray-binary conversionConfigurable
Direction detectionYes
Direction reversalConfigurable
Encoder value normalizationConfigurable
Encoder value latch functionCan be triggered with a 24 V digital input
Power supplied to encoder by module24 V DC, 100 mA, fused
DiagnosesWire break/frame error, 24 V encoder supply, Configuration error
Current draw
ExternalMax. 20 mA + load on 24 V encoder supply (max. 100 mA)
Internal max. 130 mA
Power dissipationMax. 1.0 W
Input characteristic curveType 2, EN 61131-2
Output current
Rated value 500 mA
Leakage currentMax. 0.5 mA
Short-circuit protectionElectronic
Parameter configuration length 10 bytes
Hot-swap capableYes



1 x SSI encoder interface600-320-7AA01