Push-Pull HTL: Simplicity and Universal Compatibility
The Push-Pull output (also called HTL — High Threshold Logic or Totem-Pole) is the simplest and most versatile interface for incremental encoders. Compatible with any PLC without configuration.
Working Principle
The Push-Pull output circuit consists of two transistors in complementary configuration (totem-pole):
- High state: the upper transistor (PNP) conducts, connecting the output to supply voltage (Vcc)
- Low state: the lower transistor (NPN) conducts, connecting the output to GND (0V)
This ensures an "active" output in both high and low states, unlike Open Collector which has a passive state.
Electrical Specifications
- Power supply: 10-30 Vdc (wide range, compatible with industrial 12V and 24V)
- Logic levels: high state = Vcc - 2V, low state = < 2V
- Output current: 20-50 mA per channel
- Maximum frequency: typically 100-300 kHz
- Maximum distance: 20-30 m (shielded cable)
PLC Compatibility
The great advantage of Push-Pull is universal compatibility:
- PNP inputs (sourcing): the encoder "sources" current → the PLC "sinks" it
- NPN inputs (sinking): the encoder "sinks" current → the PLC "sources" it
- Push-Pull works with BOTH without changing configuration
Wiring Diagram
- Red wire: +24V power
- Blue/black wire: GND (0V)
- Green wire: channel A
- White wire: channel B
- Yellow wire: channel Z (index)
- Shield: grounded at PLC side (single point)
- Total: 5 wires (+ shield)
Advantages
- Simple wiring (fewer wires than Line Driver)
- Wide power supply range (10-30V)
- No termination resistor needed
- Immediate compatibility with any PLC
Limitations
- Single-ended signal (not differential) → sensitive to EMC interference
- Maximum distance limited to 20-30 m
- Not suitable for strong electromagnetic interference environments
When to Choose It
Standard applications with encoder-PLC distance < 20 m, environments without significant EMC interference, installations with generic PLC of unknown input polarity.
