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Skipping a subroutine for basic pump control would save milliseconds or microseconds on program scan time. If this much scan time matters for a MicroLogix project, you’re using the wrong platform. Program organization is a fundamental reason for subroutines. There are situations where conditional routines are the logical choice. An example that comes to mind is call a subroutine at completion of a message to process data received in that message. A subroutine can also be called from a timed interrupt or a condition interrupt. A registration sensor on a packaging machine could trigger a subroutine for tracking position. I have used a subroutine triggered by PLC fault to move data to HMI registers for diagnostic information on why the PLC was crashing. I understand wanting to not execute useless code but I wouldn’t recommend it for normal operating. I recall only one time skipping a subroutine because it didn’t matter. When a HMI recipe select screen was opened, I ONS JSR'd to a subroutine that repopulated 48 recipe name strings. I did hate to execute that every scan because only the previously selected one might have changed. That’s just populating strings. I wouldn’t do it for output controls.

You could probably make your own. I would be surprised if you had a thermal mass to heat energy ratio that required heat cycles as fast as 200Hz. Have a cycling timer of a few seconds or whatever timebase works for you. Multiply 0-100 heat PID output by units that make 100% the full cycle time. If this value is less than timer accrued value, energize output. You should have a SSR or similar firing the heater. Mechanical contacts would burn up quickly.

My guess for RCAS is Resistive Capacitive Arc Snubber - used to dissipate coil collapsing magnetic field energy to extend life of contacts.

If it’s likely to get water spray and these are the choices, I would choose the 6. The display is replaceable while keeping the logic module. I've replaced several screens in a washdown environment. A logic module hasn’t failed yet. A display swap is quick and easy. I would choose the 7 for a dry or damp area. The plastic cam locks seal the unit to the panel well enough, but I don’t perceive it as secure as the screw clamps of the 6. It’s the current model and cheaper. A backup of the application on a USB stick can be copied to a new unit, provided the firmware version matches. If you’re new to FactoryTalk View for this, I expect you to quickly develop a strong dislike for it. I’m getting used to it now but it really makes me appreciate others. Consider proposing a PV800 as an additional cheaper choice to the customer, especially if using a ML1400. I like it as a simpler cheaper alternative to 6/7 where it can be used. It’s developed with free CCW. Unfortunately the largest size is 10.4”. It doesn’t have all the features like parameter passing but for simple I/O interface I think it’s great. Know how you want to connect, serial or ethernet when ordering. Either works fine. Second guessing myself...the 6 doesn’t withstand direct spray very well. Maybe the 7 would be better?