You are not registered yet. Please click here to register!

plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!

Go Back - Interactive Q & A > - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Thread Tools Display Modes
Old February 9th, 2018, 10:26 AM   #1

richleva is offline
Join Date: Sep 2012
Location: ontario
Posts: 69
Question Alalog Output Scaling, Please help !


I need to control an Acid pump at a water treatment plant, Using an analogue output of a micrologix 1400. This PLC only has 0-10V analogue output card. So I installed a 500Ohm resister across the pair of my instrument cables, to get my 4-20Ma. So at 2Votls will be my 4mA and 10 Volts will be 20mA.

PLC Resolution (0-4095)

Acid pump 1
0-60 L/H

Acid pump 2
0-12000 strokes per hour

In The PLC. I'm using RXLogix micro developer, and I'm using the SCP instruction to scale

I tried using one SCP block, but the input number that is entered is much lower than what is display in the pump



input: F8:1 --->> entered 7 l/h in the HMI

input min: 0
input MAX: 60

scale min: 819
Scale max: 4095

Output O:0/0 --->> Pump said about 12 l/h

I tried using two SCP block per pump, to scale input number from the HMI to 4-20 and the second would scale the 4-20 to the PLC 12 bit analog card (4095)

but I got similar results...


Input: F8:1 ---->> entered 15l/h in the HMI
input min: 0
input MAX: 60

scale min: 4
scale max 20
Output F8:2 ---->> 8 mA


Input: F8:2 ---->> 8 mA

Input min: 4
input max: 20

scale min: 819
scale max: 4095

Output: O:0/0 ----->> 1638

The results for example

today I entered 30 L/H in the HMI the pump said 41.8

for the other pump I entered 6000 strokes per hour, pump would say 12000 strokes per hour.

PLEASE HELP, if anyone has ideas....

Thanks everyone

Last edited by richleva; February 9th, 2018 at 10:29 AM.
  Reply With Quote
Old February 9th, 2018, 10:36 AM   #2
United States

RDS is offline
RDS's Avatar
Join Date: May 2003
Posts: 170
The 1400 analog output can only source into 1000 ohms or greater. The 500 ohm resistor is loading down the output so you will never reach full scale.
  Reply With Quote
Old February 9th, 2018, 10:38 AM   #3

lfe is offline
Join Date: Jun 2007
Location: Barcelona
Posts: 344
You can't convert 0-10V to 4-20 mA output by simple adding a resistor, you need a converter like this:

and probably it will solve the scaling too

In addition a the voltage output need to be connected to high impedance input, if not the voltage drop across the cable generate a significant error.

Last edited by lfe; February 9th, 2018 at 10:47 AM.
  Reply With Quote
Jump to Live PLC Question and Answer Forum


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Unity 7.0 on m340 Analog Output Block Scaling Problem rc10forlife LIVE PLC Questions And Answers 9 March 2nd, 2014 04:17 AM
First Program andrewt LIVE PLC Questions And Answers 48 December 23rd, 2011 08:23 PM
Siemens Step5 150u Analogue Output Scaling b16mcc LIVE PLC Questions And Answers 20 October 24th, 2011 10:06 AM
Relay Output Module vs. AC Output Module smohamed LIVE PLC Questions And Answers 10 March 19th, 2008 12:52 AM
Encoder output types Orn Jonsson LIVE PLC Questions And Answers 10 May 9th, 2007 05:25 PM

All times are GMT -5. The time now is 04:06 AM.