Cold Start vs Hot Start

Know the four start modes

The controller has four start-up methods: initial start, controlled start, cold start, and hot start. In normal daily operation the controller comes up with either a cold start or a hot start, and which one runs depends on whether the hot start function is enabled. Initial start and controlled start are maintenance modes and are not used in routine operation.

Understand what a hot start restores

A hot start is a normal power-up performed while power restoration is enabled. It sets each digital, analog, robot, and group I/O output the same as it was before the last power-down, restores the feed rate override, manual feed coordinate system, and machine lock, and brings back the program state. If a program was running right up to the last power-down, that program comes back in the pause state rather than running on its own.

Understand what a cold start clears

A cold start is a normal power-up performed while power failure recovery is disabled. On a cold start each output of digital, analog, robot, and group I/O is turned off or set to 0, the program is aborted and the beginning of the program becomes the current line, the feed rate override is reset to its initial value, the manual feed coordinate system enters the JOINT state, and machine lock is released. It gives a clean, predictable starting point without deleting saved programs or data.

Compare the two side by side

The manual's start-mode status table makes the contrast concrete: register contents are restored in both modes, but override, execution line, and I/O condition are only restored on a hot start. On a cold start the override defaults to 10%, the execution line returns to the first line, and all outputs come up off. The teach pendant returns to the screen type used at power-down after a hot start, and to the hint screen after a cold start.

Check whether hot start is enabled

Open the system config menu ([6 SYSTEM], Config) and find Use HOT START. When it is TRUE, the controller performs a hot start at power-on; when it is FALSE, the controller starts in cold start mode instead. The default setting is TRUE.

Understand the I/O power fail recovery setting

The I/O power fail recovery field controls how outputs and simulated states are handled at power-up. The four modes are NOT RECOVER (all outputs off, simulated state reset), RECOVER SIM (simulated state recovered, actual outputs and simulated I/O off), UNSIMULATE (recovery performed but simulated states reset), and RECOVER ALL (outputs and simulated states restored to their pre-shutdown values when hot start is enabled). This setting is why two robots with the same hardware can behave differently after a power cycle.

Understand program selection after power-on

When hot start is enabled, the program that was selected at power-off is selected again as it was. When hot start is disabled, program selection depends on the Restore selected program field: TRUE reselects the previously selected program, and FALSE leaves no program selected. Restore selected program is TRUE in the standard setting.

Perform a routine hot start

With Use HOT START set to TRUE, simply turn the controller power breaker to ON. After a few seconds the teach pendant returns to the screen that was displayed at power-off, and the restored program state is available. Confirm the robot is in a known, safe state before doing anything at speed.

Force a cold start when hot start is enabled

To get a clean start even though hot start is enabled, hold the [PREV] and [NEXT] keys while you turn the power breaker to ON. The CONFIGURATION MENU appears with 1 Hot start, 2 Cold start, 3 Controlled start, and 4 Maintenance. Select 2 Cold start, and the controller finishes powering up in cold start mode.

Perform a cold start when hot start is disabled

When Use HOT START is FALSE, every normal power-up is already a cold start. Turn the power breaker to ON and the system starts up in cold start mode without any key combination. The utilities hint screen appears once the cold start completes.

Know when a setting change forces a cold start

Some configuration changes only take effect after the controller is cycled, and the controller performs that restart as a cold start automatically. Examples include changing the Detect FAULT_RESET signal setting and changing the Use PPABN signal setting, both of which cycle power and come back cold. When a joint operating area upper or lower limit is changed, the new limit takes effect after a cold start.

Understand controlled start and reaching cold start from it

Controlled start loads a simplified system menu that cannot operate the robot but can change system variables that are normally protected, read system files, and set up the robot. Reach it by holding [PREV] and [NEXT] at power-on and selecting 3 Controlled start from the configuration menu. To return to normal operation, press [FCTN], then select 1 START (COLD) to perform a cold start.

Understand initial start and why to leave it alone

Initial start deletes all programs, resets every setting to its standard value, and erases the factory-set mastering data, then automatically drops into a controlled start. It is reached by holding F1 and F5 at power-on to enter the BOOT MONITOR and selecting the INIT start option. This is a last-resort maintenance action for a main board or software replacement, so route it to qualified personnel and confirm a full backup exists first.

Caution: An initial start erases mastering and all data, so the robot cannot position correctly afterward until it is re-mastered. Do not perform an initial start to clear a fault; make a backup and involve qualified maintenance personnel first.

Confirm the controller state after the start

After a cold start, remember that machine lock is released and the override has reset to its initial value, so the robot can move and the speed may differ from what you expect. Check the active program, the cursor line, and the mode switch before enabling motion. Verify the robot's actual position matches what the controller expects.

Reselect and verify the intended program

If no program came up selected, or the wrong one did, open the program list and select the correct production program. Confirm the cursor sits at the expected line, since a cold start moves the current line to the beginning of the program. Do a low-speed check before running at full override.

What can go wrong: hot start did not restore the outputs

Even with hot start enabled, all I/O outputs come up off instead of restored when the I/O allocation was changed before power-off, when an I/O unit was mounted or removed, or when the number of signals was changed on the I/O Link screen. A blown I/O unit fuse or an I/O unit that lost power causes the same result. If expected outputs are off after a hot start, check the I/O configuration and unit status rather than assuming the hot start failed.

What can go wrong: controlled start cannot be selected

Controlled start needs sufficient free FROM to run, and it cannot be selected from the configuration menu if less than 600KB of FROM remains. Check the available FROM on the memory status screen. If it is short, remove unnecessary data from the FR: or FRA: device to make room before attempting the controlled start.

What can go wrong: program came up unselected or paused

After a cold start with Restore selected program set to FALSE, no program is selected, which can look like lost data when it is only the expected cold-start behavior. After a hot start, a program that was running at power-down returns in the pause state, so pressing start would resume mid-cycle. Confirm which program is active and where the cursor sits, then verify robot position before continuing.