TON, TOF or RTO for a machine hours meter?

RTO, and it is not close. A TON resets its accumulated value the moment the rung goes false. For a running hours meter that means every time the machine stops you lose the count, which is the opposite of what the meter is for. RTO is retentive: it holds the accumulated value when the rung goes false and carries on from there when it goes true again. You clear it deliberately with an RES, which is exactly the behaviour you want for a total that spans shifts. Two things to get right: - Put the RES somewhere an operator cannot reach by accident. A maintenance reset behind a key switch or a password level. - Decide what happens on a power cycle. RTO accumulators are usually retentive across a restart, but confirm it on your platform rather than assuming, because it varies. If you are on a platform with no RTO, add the elapsed time into a retentive DINT yourself on a one second pulse. Slightly more work and completely portable.