Stäubli CS8C/CS9 — ESB1-2 Safety Chain Open

Stäubli · intermediate · 25 min

A Stäubli ESB1-2 error means part of the external safety chain is still open, or the two redundant safety channels disagree. Find the open input in the cell wiring before you can re-enable the arm.

⚠️ ESB1-2 is a safety-chain fault — the controller is correctly refusing to power the arm because a safety input is open or its two channels disagree. Fix the wiring/device, never jumper the chain.

What ESB1-2 means

On a Stäubli CS8C / CS9 controller, ESB1-2 indicates that at least one part of the external safety chain is still 'open'. The Stäubli safety system uses two redundant channels, and it flags ESB1-2 when one safety channel is high and the redundant channel is low — i.e. the two channels disagree, or an input in the chain has not closed. Until the chain is complete and both channels agree, the controller will not allow motor power.

What's in the safety chain

The CS8C/CS9 external safety integrates several inputs, any of which can hold the chain open:

  • Emergency stops on the Manual Control Pendant (MCPES) and the Working Mode Selection front panel (WMSES).
  • User Emergency Stop (UESA) and User Safety Stop (UESB) signals from the cell.
  • The cell door emergency/guard input (DOOR).
  • The automatic (COMP) and manual (MANU) mode channels.

A pressed E-stop, an open guard door, or a miswired/half-made UESA/UESB connection are the usual causes.

Step-by-step recovery

  1. Read the exact fault on the pendant and the controller's 7-segment display. ESB1-2 confirms a safety-chain problem; note anything else displayed.
  2. Release every emergency stop — the pendant (MCPES), the front-panel WMSES, and any external UESA station. Twist to release pressed buttons.
  3. Close the guard / door (DOOR input) and confirm the guard switch is genuinely made.
  4. Check the redundant channels. ESB1-2 specifically appears when one channel is high and the other low. Inspect the external safety wiring on the UESA/UESB (and mode) inputs for a single-channel fault — a broken wire, a half-inserted connector, or a relay/contact that closes one channel but not the other. Both channels must switch together.
  5. Verify mode selection (COMP/MANU via WMSES) is valid for what you're doing.
  6. Re-enable the arm. With the chain complete and both channels agreeing, enable the motors — on CS9 power the motors after enabling with the button on the SP2 teach pendant.

If the pendant is unresponsive

If the SP2 pendant itself is frozen while you diagnose, you can reboot the SP2 on CS9 by pressing Home + Back + Legacy Mode simultaneously.

Verify

Once both safety channels are closed and agree, ESB1-2 clears and motor power can be enabled. If ESB1-2 returns immediately, one channel is still open — recheck the UESA/UESB wiring for a single-channel fault.

Never do this

Do not bridge or jumper a safety channel to clear ESB1-2. The redundant-channel check exists to detect exactly the single-channel faults you're looking for; defeating it removes the protection.

Key points

  • ESB1-2 = part of the external safety chain is still open, or the two redundant safety channels disagree (one high, one low).
  • Chain inputs include MCPES/WMSES E-stops, UESA/UESB signals, the DOOR guard, and the COMP/MANU mode channels.
  • Release all E-stops, close the guard, and confirm valid mode selection.
  • For a channel mismatch, inspect UESA/UESB wiring for a single-channel fault — a broken wire, half-made connector, or a contact closing only one channel.
  • On CS9 enable motors with the SP2 button; reboot a frozen SP2 with Home+Back+Legacy Mode. Never jumper a safety channel.

Codes and symptoms

ESB1-2 — External safety chain open
At least one safety input is open, or the two redundant channels disagree (one high, one low); motor power is inhibited.