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Pulse & direction control
Pulse/direction servo control: electronic gear, wiring and lost pulses
In pulse/direction control every pulse tells the drive to move one position step and the direction signal tells it which way. It is a simple interface that works with every PLC and motion card, and with the right electronic gear ratio and noise-proof wiring it is precise and reliable.
6 questions answered
Basics
Commissioning
Troubleshooting
Basics
01
What is pulse/direction control, and when is it the right choice?
In pulse/direction control the controller sends the drive one position step per pulse, and a separate direction signal sets which way it moves; speed comes from the pulse frequency and distance from the pulse count. It is the right choice for a few independent axes, with an existing PLC's pulse outputs, when you want to commission quickly without setting up a network. The SD-LP series supports position control by external pulse.
Pro: simple, universal, works with any controller
Con: no status or alarm data back from the drive; the wiring is noise-sensitive
What is the difference between pulse+direction, CW/CCW and A/B phase modes?
There are three common pulse formats. Pulse+direction: one line carries pulses, the other a direction level. CW/CCW: pulses on one line move forward, on the other backward. A/B phase (quadrature): two square waves 90° apart, with direction set by which phase leads. The drive's pulse-mode parameter must match what the controller produces.
Typical sign of a mode mismatch: the motor turns one way only, or chatters
In A/B mode the drive may count ×4; account for it in the gear ratio
The electronic gear ratio sets how many encoder counts one command pulse is worth. First choose how many pulses you want per motor revolution, then enter the ratio as encoder resolution divided by that number. Example: for 1 µm resolution on a 10 mm-pitch ball screw you need 10,000 pulses per revolution; with a 17-bit encoder (131,072 counts) the ratio is 131072 / 10000.
Keep numerator and denominator within the range the manual allows
Check the pulse frequency needed: 3000 rpm × 10,000 pulses/rev = 500 kHz
That frequency must be within both the controller output and the drive input limit
Should I use line-driver (differential) or open-collector pulse outputs?
Use a line driver where you can. A differential signal carries noise equally on both wires, where it cancels, and allows higher pulse frequencies and longer cables. Open collector is simpler but exposed to noise: keep the cable short and, with a 24 V supply, add the series resistor the drive input needs for its rated current.
Confirm in the manual whether the drive input expects 5 V or 24 V
Run the pulse cable as shielded twisted pair, away from power cables
An axis driven by pulses slowly loses position. Why?
A position error that builds up means the drive counted a different number of pulses than the controller sent. Extra pulses come from noise; missing ones from pulses too short for the drive to detect, or from the direction signal changing right before a pulse. First compare the drive's command-pulse counter with the controller's count.
The motor travels two or four times the distance, or turns the wrong way.
If the distance is an exact multiple, the cause is almost always the pulse mode or the gear ratio: ×4 counting in A/B mode, or the drive expecting pulse+direction while the controller sends CW/CCW. For a reversed direction, flip the logic with the drive's rotation-direction parameter rather than rewiring.
