from the Instrumentation department...
polysonics Ultrasonic flow meter
Continuous process industries, DCS questions. topic
Posted by Giri on 22 February, 2003 - 12:15 pm
Recently i had a problem with Polysonic flow transmitter(model DCT 1088).The transmitter works on 24 v DC and is powered from the PLC power supply.Its output is 4 to 20mA.The transmitter has a serial port , through which we can configure the transmitter using Laptop/Notebook.The problem is whenever I try to hook up the Laptop/notebook to the serial port(9 pin)of the transmitter,it is causing the PLC control supply to trip ,causing the whole system shutdown.
(The grounding connections and other wirings in the transmitter are O.K.)

Can any one suggest what may be the possible reasons for the above problem?.

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Thx and Regards..Giri.

Posted by Anonymous on 24 February, 2003 - 12:49 am
Sounds like a short circuit, try making a cable up for use between laptop and flow meter, try only connecting pins 2,3,5 at both ends (2,3 may need to be crossed). Some manufacturers will use the RS232 port connection to power hand held programmers by using "spare" pins, you may find your exisiting RS232 cable is bridging this unoffical power supply.

If fault still occurs place a volt meter between the metal body (shell) of both the RS232 cable and the RS232 flow meter without connecting together, if a voltage is showing greater than a few mV then you have major problems.

Posted by Steve Myres on 24 February, 2003 - 12:56 am

Most of the internal PLC supplies I've seen don't have a lot of capacity, maybe driving the transmitter output at the same time as the port circuitry is just too much current draw. If so, can you power the transmitter from a dedicated PS, either permanently, or just during configuration?

My other thought was that an optical RS232-RS232 isolator/coupler between the transmitter and laptop might cover some sins on the part of the designer of one or the other of the com ports in question.

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