What causes robot powers up but won't enable, srdy stays off?

Another alarm fired at the same time and is the real cause; Loose connector on the E-stop board or servo amplifier; Instant disconnection of the incoming power source; Wrong setup after adding or changing an auxiliary axis; Machine lock or disconnect switch configuration on multi-group systems; Hardware fault in the E-stop unit or servo amplifier

Where should I start with robot powers up but won't enable, srdy stays off?

Open the alarm history and look for any alarm that fired alongside SRVO-021. If one is there, work that alarm first. Check whether an instant power disconnection occurred. Confirm the E-stop board and servo amplifier connectors are securely attached: CP5A, CRMA92, CRMB22, CRMA91, and CXA2A/CXA2B on aux-axis setups. If this started right after auxiliary-axis changes, re-verify the FSSB line number, hardware start axis, amplifier number, and amplifier type. On multi-group systems, check machine lock status and the disconnect switch setting ($ROBOT_ISOLC). If the settings and connections are correct, treat it as a hardware problem. See SRVO-021.

Power

Robot powers up but won't enable, SRDY stays off

The controller has power, but the servo amplifier's magnetic contactor never closes when commanded, so the drives never energize.

Most likely causes

01Most likely

Another alarm fired at the same time and is the real cause

SRVO-021 often rides along with the fault that blocked servo power. Open the alarm history and clear that companion alarm first.

SRVO-021

02Very likely

Loose connector on the E-stop board or servo amplifier

A loose connection can keep the contactor from closing. Confirm CP5A, CRMA92, CRMB22, CRMA91, and (on aux-axis setups) CXA2A or CXA2B are securely attached.

SRVO-021

03Common

Instant disconnection of the incoming power source

A momentary drop in incoming power can post this alarm. Check whether an instant disconnection occurred.

SRVO-021

04Worth checking

Wrong setup after adding or changing an auxiliary axis

If it started right after aux-axis work, the FSSB line number, hardware start axis, amplifier number, or amplifier type may be set wrong.

SRVO-021

05Less common

Machine lock or disconnect switch configuration on multi-group systems

Machine lock can hold servo power off. Check the machine lock status and the robot disconnect switch setting ($ROBOT_ISOLC); a failed disconnect switch unit can also trigger this on an auxiliary axis.

SRVO-021

06Less common

Hardware fault in the E-stop unit or servo amplifier

Once the settings and connections check out, the manual points to replacing the E-stop unit or the servo amplifier.

SRVO-021

Where to start

Open the alarm history and look for any alarm that fired alongside SRVO-021. If one is there, work that alarm first.
Check whether an instant power disconnection occurred.
Confirm the E-stop board and servo amplifier connectors are securely attached: CP5A, CRMA92, CRMB22, CRMA91, and CXA2A/CXA2B on aux-axis setups.
If this started right after auxiliary-axis changes, re-verify the FSSB line number, hardware start axis, amplifier number, and amplifier type.
On multi-group systems, check machine lock status and the disconnect switch setting ($ROBOT_ISOLC).
If the settings and connections are correct, treat it as a hardware problem. See SRVO-021.

Still stuck? Start with the closest matching alarm reference.

Related alarm codes

SRVO-021SRDY off

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