Ack! lightning

[sequimscada]

A while back I posted a question as to why, in about 1985, the water department chose to convert 4-20mA to PWM.
I just found the answer. In another system we have two controllers, about 1/2 mile apart, talking to each other via RS-485. I did not know that their RS-485 ports were not isolated! We recently had a lightning strike to the earth nearby. The ground potentials between the two sites differed due to this strike. It blew the tops off of some ICs, protection diodes, etc. So, RS-485 opto-isolators are ordered.

This explains the previous mystery: why they chose PWM for their site-site 4-20mA loop. We were wanting to remove the 4-20mA <-> PWM convertors/relays, etc. from both ends. I guess we'll have to add 4-20mA isolators at both ends or leave it as is.

 

[djs]

In central Florida it is very common to use optical fiber for any runs outside a building. We are after all the lighting capital of the US (but not the world).

 

[IRstuff]

No kidding, there was a thunderstorm just about every day of last week, when we went to Disneyworld.

However, it did make the crowds really thin down at Blizzard Beach on Tuesday ;-)

TTFN

FAQ731-376