Turbine flowmeter interface to a PLC

[Malcolmt]

Can anyone offer advice on interfacing turbine flowmeters to a PLC.
We are building several filling lines for small volumes..50ml. The fluid is metered by turbine flowmeters, these are not my choice but dictated by others. The signal from the meter mag pickup is amplified by a Red Lion preamp, the distance from the meter to the PLC is 50 meters max, the frequency of the signal from the meter will be around 500hz.
We are using AB 1771-VHSC counter modules to count the pulses and control the filling valves. My diagram shows a very simplified circuit of the PLC module input.
My questions are…
1 Is it better to mount the load resistor at location 1 in the PLC panel or location 2 close to the preamp ? Can anyone explain the pro’s and cons of each option. My opinion is location 1 , the pulses to the PLC are generated within the panel. If the resistor is mounted at location 2 then the current pulses could be affected by interference along the cable.
2 What cable configuration is best to connect back to the PLC. Existing installations use 2x twisted pairs with the preamp output and 0v line in one pair and the 24v supply using half of the second pair. I’m under the impression a twisted pair will only cancel noise efficiently if the signal in each line is equal such as a 4-20ma two wire loop or balance data comms signal such as RS-485. A twisted pair may not help in this situation.
3 Is a 500hz signal likely to be attenuated by a general screened cable intended for mA instrumentation signals ? Would it be better to use a cable intended for data comms.. such as RS-485 ??
Thanks in advance,
Malcolm(t)
Johannesburg, South Africa.

 

[iant]

The Load resistor is a 'Pull Up' resistor designed to increase the voltage at the PLC input. the location at the PLC is probably better. this will allow for the measurement of the pulse.

500Hz is an extreemly slow pulse rate (500/sec)
The high speed counters are designed for 200 times that speed.
so there will be no problems.
The cable size is not an issue with the pull-up resistor allowing for voltage drop.
Calculate your potential voltage drop over the cable length (x2)
Then size your cable.
Be aware of the need for screening as other power cables may cause issues.

If you run RS485 cabling you could change it to RS485 at a later date - Also change the redlion to a redlion - CAB5 unit - this will give you local display if you wish.