Here it is Friday, December 21, 2007 about 2:30pm, while most process safety specialists and marketing managers are enjoying the tidings; I just had to get something off my chest before I begin my holiday break. 

I was hoping to make my 2008 Process Safety predictions but I find myself still pontificating on what next year will bring.  So we’ll table that discussion (for now), and I’ll share with you another interesting discussion I just had.

A gentleman from the ISA safety list was inquiring about the speed of response for analog devices over Profibus/profisafe.  At first he was concerned if the response time was sufficient for his high speed requirements.  I’m not sure he liked my answer, because he kept responding that it wasn’t good enough.  So what was good enough I inquired? 

It’s been understood for years that pure electrical signals (i.e. 4-20ma) travel near the speed of light while a digital protocol is tracked at a specific baud rate (no where near the speed of light).  No one would argue that, however I just wanted to make certain we also considered the device itself. 

I went on to explain that a discrete device takes about 10ms to get the signal “on the wire” where an analog device is closer to 200ms.  Now if you want to consider using digital communication, you will need to add the lag time for that as well.  Profibus PA has about a 10ms overhead with an additional 10ms per device on the segment.  So for example, if you wire 10 devices on a PROFIBUS drop, you can estimate that it will take 110ms response time on the wire.    Now you add the device time (200ms) and the bus time (110ms) and it takes a little over 300ms for the signal to reach your control system.

Of course that’s just the half of it, because now your control system has to process the signal and drive their outputs (and that could take SECONDS).

So the choice should be easy, if the BUS isn’t fast enough, go Old School.

Merry Christmas and Happy New Year to all.

Charlie