Agilent 66309D: Using INHibit input on rear panel - Page 1
Forum section: electronics / testgear

author: IDEngineer
author_rank: Super Contributor
timestamp: October 20, 2019, 10:20:20 pm
content: I'm still learning all of the features of my new-to-me 66309D. Over the last couple of days I've been using its INH input on the rear panel to remotely switch the output voltage on and off. This is incredibly useful for long-term power cycling tests, which is exactly how I'm using it. But a display weirdness has cropped up that I can't explain.I programmed a function generator to emit a 10 second period pulse train with a nine second active high period (90% duty cycle) and connected that output to the 66309D's INH input. Worked perfectly. The DUT was powered up for nine seconds (which allowed it to complete its bootup process), then powered down for one second, lather rinse repeat. I let it run this way for 48 hours and everything went great. When the INH signal was high, the 66309D's display showed the programmed voltage and consumed current; when the INH signal was low, the display dropped to ~zero V and A.Here's the odd thing: During this 48 hour test, the ONLY display indication of INH activity was the displayed voltage and current changing.Today, with that test done, I formalized the connections to the somewhat inconvenient to reach INH connector since I expect to be running tests like these quite often. I did so, hooked everything up, and the 66309D again worked as intended... but now when the output is turned off I get a "Prot" indication on the display in addition to the voltage/current dropping to ~zero.I am 99.999% certain this "Prot" indication wasn't there during the 48 hour test. The reason I believe that is because when I saw it today, I presumed I'd hooked up something wrong and the unit was protecting itself. If I'd seen this a couple of days ago, I'd have investigated it then. However, extensive tests and lots of time in the user manual have revealed that one of the reasons "Prot" is displayed is when the INH input is asserted low (or even just shorted to ground, its quiescent state is pulled to +5VDC as measured at the INH pins). Fine, but why wasn't it being displayed during the 48 hour test?If you press the Protect button on the front panel the unit reports why "Prot" is being displayed. Sure enough, it reports "RI ASSERTED" (RI = "Remote Inhibit"). And this is what the docs say should happen.As I said, that "Prot" warning wasn't there a couple of days ago. The only reason I have 0.001% doubt is because everything I'm reading and testing suggests the unit appears to be working properly today, which naturally makes me question if I somehow missed that big, obvious "Prot" for 48 straight hours. I seriously doubt it... I was carefully watching the displayed voltage and current change as the function generator controlled the power supply, and "Prot" is in perfect sync with those changes. Since "protection" suggests something is connected wrong - particularly with an intelligent supply like the 66309D - I would have immediately dug into it (just like I did today) on the presumption that a sense lead had come disconnected, or the secondary supply output leads were shorted, or something.Bottom line: I cannot imagine that I missed the "Prot" display flashing on and off. I honestly do not think it was active. Yet today I can't get rid of it, and cannot find an option to disable it for those use cases (like this one) where asserting INHibit really isn't a fault at all.Help a guy regain his sanity... does anyone know of a situation or configuration where the 66309D can use its INH input without displaying "Prot"?!?