ML 1761 Checking Analog Signal

[Oceanamie 0]

Hello, all! I'm new to PLC programming (they don't teach you it in college!), and I'm trying to program a MicroLogix 1000 unit, specifically a 1761 with 20 analog inputs. I need help knowing how to program the unit. I have 8 analog signals, 0 - 5 V DC, and I'd like to know how to check if the analog signal is 5 volts or even above 5 volts. So if the signal at the analog input is 5 volts or above, then the output will send a signal (which will actually connect to a switch to turn off a power supply). If someone can help me with this, that would be great. Also, does the unit record data?

[Ken Roach 194]

I think you might have a misunderstanding of the I/O circuit count on those MicroLogix 1000 controllers. The "20-point MicroLogix 1000 with Analog" has 12 discrete inputs, 8 discrete outputs, 4 analog inputs, and 1 analog output. Two of the analog inputs are Voltage type, and two are Current type. Determine exactly which model of MicroLogix 1000 you have (there will be a part number like "1761-L20BWB-5A") and download the User Manual for an example of how the analog inputs are wired. In general and Analog input channel will place an integer value into one Word of the Input data image. The value isn't in volts: instead it's in counts over some range. A 12-bit analog input will have counts betwee 0 and 4095 that are proportional to 0 - 10 V on the analog input channel. For example the Channel 0 voltage input will be addressed as 'I:0.4". The range of the Voltage inputs is 0 to 10.5 volts = 0 to 32767 counts. So if the voltage input is above 5.0 volts, it will be above about 15603 counts. In basic ladder logic, you can use the "GRT" instruction to determine when the Input is above the threshold you want.

[Oceanamie 0]

Ken, Thanks for the clairifcation and help. I should be able to put the 0 - 5 V DC input into the discrete input though, correct? I'll refer to the manual that you have recommended regardless to read and learn. Is it possible to show me an example of one input within ladder logic? If an input is above the 15603 counts, then it send a signal to 'switch' a relay? How would the output signal be, is what I'm asking to learn I think.

[Mickey 71]

No, a discrete input needs a minimum of 14 volts to turn on, see picture .. Edited 6 Mar 2014 by Mickey