20-750-S3 — 755 Series Safe Stop Option Card, SIL 3 / PLe

The 20-750-S3 is a safety option card for the 755 series AC drive, providing Safe Torque Off (STO) and Safe Stop 1 (SS1) functions rated to SIL 3 (IEC 62061) and PLe Category 4 (ISO 13849-1). It installs in a dedicated safety option slot on the 755 drive and adds hardwired safety inputs that are processed by the card's independent safety processor — separate from the drive's main control electronics and unaffected by drive faults or communication failures.

The distinction from the basic hardwired STO already built into the 755 is significant: the 20-750-S3 achieves a higher integrity level (SIL 3 / PLe vs. SIL 2 / PLd), adds SS1 functionality for controlled deceleration before STO engages, and provides a certified feedback output confirming the safety state has been achieved. For applications requiring the highest safety performance levels — robotics, metal forming presses, hazardous material handling — the dedicated safety option card is the appropriate solution.

Specifications

Parameter	Value
Part Number	20-750-S3
Compatible Drive	755 Series AC Drive
Safety Functions	Safe Torque Off (STO), Safe Stop 1 (SS1)
Safety Rating	SIL 3 (IEC 62061), PLe Category 4 (ISO 13849-1)
STO Input Channels	2 (dual-channel, independently monitored)
STO Input Voltage	24V DC
STO Response Time	≤ 8 ms
SS1 Stop Type	Ramp deceleration to zero speed, then STO
SS1 Deceleration Monitor	Yes — speed ramp monitored against time/deceleration limits
Feedback Output	Safety status relay (confirming STO active)
Discrepancy Detection	Yes — monitors dual-channel input agreement
Installation	755 drive safety option slot
Standards	IEC 62061 (SIL 3), ISO 13849-1 (PLe Cat 4), CE, UL

STO vs. SS1 — Choosing the Right Stop Function

STO (Safe Torque Off) immediately removes gate pulses to the drive's output transistors. The motor coasts — it generates no torque and decelerates under friction and gravity only. This is IEC 60204-1 Stop Category 0: immediate power removal. It is appropriate where the coasting behavior doesn't create a hazard — horizontal conveyors, fans, and most pumps.

SS1 (Safe Stop 1) initiates a controlled deceleration via the drive before STO engages. The motor actively brakes to zero speed over a programmed ramp, then STO fires to inhibit gate pulses. This is Stop Category 1. For vertical axes, high-inertia loads, or processes where an uncontrolled coast creates secondary hazards, SS1 prevents those outcomes while still achieving a certified safe stop. The 20-750-S3 monitors the deceleration ramp — if the motor doesn't decelerate within the configured envelope (speed vs. time), the card fires STO immediately as a fallback.

FAQ

Q: What is the difference between the 20-750-S3 and the standard hardwired STO on the 755?

The standard 755 STO provides SIL 2 / PLd. The 20-750-S3 adds a dedicated safety processor achieving SIL 3 / PLe, adds SS1 controlled deceleration, and provides a certified feedback relay. Use the standard STO where SIL 2 / PLd is sufficient; use the 20-750-S3 where the risk assessment requires SIL 3 / PLe or SS1 capability.

Q: Can SS1 be used without STO as a standalone stop function?

No. SS1 on the 20-750-S3 always culminates in STO — the controlled deceleration is followed by gate pulse inhibit. SS1 is a timed transition to STO, not an independent stop function.

Q: Which slot in the 755 chassis does this card occupy?

The 755 has a dedicated safety option slot separate from the standard option card slots. The 20-750-S3 installs in this safety slot. Consult the 755 option card installation documentation for the exact slot location — it varies by drive frame size.

Q: Does the 20-750-S3 require a safety PLC, or can it be driven from a standard safety relay?

Either works. The card's inputs are hardwired — they accept 24V DC from a safety relay's force-guided contacts or from a safety PLC's hardwired safety output. The choice depends on the overall safety architecture; the card's certified performance level is independent of the input source type.

20-750-S3 New In Box 20750S3 Option Module