There are two inverters inside a 256, one for Tig and stick, and a separate one for plasma. And oneboard to rule them all, and several shared analog circuits (such as the high frequency on the older units) which are hard to optimize for one or the other (speaking as a layman to electronics). Things are more tightly spaced, and there are a lot more parts inside period. Where you have more parts, you have more potential for failure. Not saying anything is going to HAVE to fail, but rather the potential exists.

As Rambozo explained, duty cycle on our units is measured in percentage of time out of 10 minutes. It's also measured on our units at maximum output, though some companies like to fudge really badly and give a lower amperage value to improve their duty cycle statistics. You'll hardly ever find a MIG's duty cycle rated at full amperage, though most of ours are.

The lower the output, as he said, the greater the duty cycle. I've tried to explain duty cycle in different places, and always ends up with an argument about what 100% duty cycle means.

It doesn't mean that at 100% duty cycle level the unit will weld indefinitely. It means that it will weld 100% of the 10 minute length of time without needing a break...That doesn't mean it won't need one at the 11th minute, though it practically could be depending upon actual amp setting etc. It's just a theoretical threshold, and it could actually be less or more.

I was perusing a competitors manual the other day for comparative information, and one of the sections discussed what to do IF the duty cycle limit was hit. It said, to allow the unit to cool for 15-20 minutes...well you do the math. I recommend 10 minutes of cooling if duty cycle threshold has been reached, but well maybe we should up our figures too.