Using one timer’s accumulated value as another timer’s preset can capture an operator-taught duration or hand off timing between machine phases. The MOV instruction is simple; the behavioral contract is not. A design can fail when time bases differ, the destination is updated continuously or an unexpected zero reaches the preset.

The safest timer handoff captures a deliberate event, validates the value and documents the units before writing the destination preset.

Define what the transfer means

Decide whether the source accumulator represents a learned duration, elapsed production time or a temporary measurement. Then specify exactly when it becomes authoritative: on a one-shot after a teach cycle, on operator confirmation or at a state transition.

Writing the destination preset every scan couples two timers continuously. That may be intended, but it can also make the active timer’s completion point move while it is timing. A one-shot transfer to an intermediate integer is usually easier to audit.

Check the data before the MOV

Units and time base

Timer structures store integer values associated with the timer time base. Confirm that the source and destination represent the same units. If conversion is required, perform it explicitly and document rounding.

Range and process limits

Validate the captured value against an engineering minimum and maximum. A zero or extreme preset can produce an immediate done bit or an unsafe delay. Add an out-of-range alarm instead of silently accepting every accumulator.

Retentive expectations

A retentive timer, a non-retentive timer and a stored integer behave differently through rung transitions and power cycles. Specify what must survive a stop, fault or download. The correct pattern follows the machine state model, not merely the instruction syntax.

A test that operators can understand

Run minimum, normal and maximum teach cycles. Interrupt each at the moment of capture, during the destination timing period and across a power cycle if retention matters. Display the learned value and validity state on the HMI so maintenance can distinguish configuration from a stalled sequence.

Editorial view: teach functions need governance

Any function that converts observed motion or time into a future setpoint is a small commissioning system. Protect it with permissions, bounds, an acceptance step and a known default. Engineers maintaining legacy platforms may also need the SLC communications guide; replacement and spare options are listed in the Allen-Bradley collection. Rockwell Automation’s SLC 500 Instruction Set Reference should be used for processor-specific timer behavior.

Questions engineers ask before the outage

Can MOV write directly to a timer PRE element?

Yes on supported SLC data structures, but the source value must be validated and its units must match the destination timer’s intended time base.

Why use a one-shot for the transfer?

It captures the value once at a defined event and prevents the preset from moving every scan unless continuous tracking is explicitly required.

What happens if the preset becomes zero?

The timer may complete immediately when enabled. Treat zero as a designed case or reject it with a minimum limit and alarm.