Detailed Explanation of 4 Encoder Output Types! Line Driver, NPN Open Collector, Push-Pull & Voltage Output Selection Guide

1.Standard Official Terminology

  1.Line-driver: Long-line differential output (Compliant with RS422 standard)

  2.NPN open collector: NPN open collector output

  3.Push-pull: Push-pull output (Complementary symmetric output)

2. Working Principle, Wiring, Pros & Cons and Application Scenarios

2.1 Line-Driver (Long-Line Differential Drive, RS422)

• Circuit Principle: Built-in differential chip, outputs two sets of complementary differential signals: A, /A, B, /B, Z, /Z.
• Signal Level: Standard 5V differential signal with a differential voltage range of 1~5V.
• Wiring Requirement: Twisted pair cable is mandatory. A 120Ω terminal resistor is required at the receiving end for impedance matching.
• Features: ✅ Excellent anti-interference performance, supports long-distance transmission (hundreds of meters) ✅ Supports high-speed pulse transmission up to MHz level ✅ Differential signal eliminates electromagnetic interference, ideal for harsh environments with inverters and motors ❌ Requires more wiring and higher cost; standard PLCs need dedicated differential receiving modules
• Applications: Servo encoders, long-distance industrial equipment, industrial sites with strong electromagnetic interference

2.2 NPN Open Collector Output

• Circuit Principle: Only the transistor collector is led out without internal pull-up power. An external pull-up resistor connected to the positive power supply (5V/24V) is mandatory.
• Signal Level: High level in standby state, pulled down to 0V when conducting. It can only sink current and cannot actively output high level.
• Voltage Compatibility: Compatible with 5V / 12V / 24V PLC systems. The operating voltage is determined by the external pull-up power supply.
• Features: ✅ Flexible voltage adaptation, compatible with most mainstream PLCs ✅ Multiple signals support wired-AND parallel connection ❌ Low maximum operating speed, vulnerable to interference in long-distance transmission; additional resistors increase wiring complexity
• Applications: Short-distance ordinary sensors, low-speed encoders, traditional PLC equipment

2.3 Push-Pull Output

• Circuit Principle: Adopts internal complementary NPN and PNP transistor structure, which can pull the signal down to 0V and actively output stable high level.
• Signal Level: Fixed factory-set operating voltage (5V or 24V). No external pull-up resistor is required.
• Features: ✅ Simple wiring, directly connect to PLC IO ports without auxiliary components ✅ Strong driving capability with stable high and low level output ❌ Output terminals cannot be paralleled; fixed working voltage; poorer long-distance anti-interference performance than differential line driver
• Applications: Short-distance industrial equipment, servo IO terminals, medium and small encoders, MCU signal acquisition

2.4 Voltage-Output (Single-Ended Voltage Output)

• Circuit Principle: Built-in voltage regulation circuit outputs standard 0~5V or 0~24V stable high and low level signals.
• Difference from Push-Pull: Lower output driving current, focusing on accurate voltage signal transmission rather than load driving.
• Features: ✅ Pure and stable signal, suitable for direct voltage reading by instruments and acquisition cards ❌ Weak load capacity, not applicable for long-distance transmission, poor anti-interference, cannot drive heavy loads
• Applications: Laboratory data acquisition, precision instruments, short-distance pulse signal collection

3. Core Parameter Comparison Table

4. Quick Selection Rules

1. Long transmission distance and strong interference environment → Line-Driver Differential Output
2. Matching 24V PLC, multi-device parallel connection and cost limitation → NPN Open Collector Output
3. Short-distance wiring and servo system matching → Push-Pull Output
4. Connecting to acquisition cards, laboratory equipment for pulse voltage detection → Voltage Output

5. Common Misoperations & Precautions

1. Open collector outputs include NPN and PNP types, among which NPN is the most commonly used. Never connect the output directly to the power supply; an external pull-up resistor is indispensable.
2. Parallel connection of multiple push-pull outputs is strictly prohibited, which will burn out the internal output chip.
3. Do not reverse the positive and negative differential signals of the line-driver. The terminal resistor shall only be installed on the receiving side.
4. Voltage-output has weak load capacity. It can only be connected to high-impedance input ports and cannot directly drive loads such as relays.