DL405 Reading Analog Inputs - DirectSOFT405

Hi All,

I'm brand new to DirectSOFT 5, and fairly new to PLC programming in general, so go easy. I was handed a program with no information on the rack configuration (I'm trying to get that now), but I had some questions about how the PLC handles analog input.

In the attached picture, it appears to me that an input bit closes, and the PLC loads the raw analog value into the accumulator. The subsequent MULB, DIVB and ADDB blocks scale the input, and the desired value is copied from the accumulator to V-memory where it is read by the SCADA system.

Questions:

1) How does the PLC know which input to read? Is it just smart enough to know that when X15 = 1, read a certain analog input and put it into the accumulator?

2) Is the PLC also smart enough to clear the accumulator to make way for the next input?

I apologize profusely for my ignorance, and thank you in advance for your help.

There would have been a point, probably in the rung just before these, where the actual binary raw value was loaded into the accumulator. Look for an LDF command. The scaling is slightly different based on the input (X15 or X16). I'm guessing that the analog card is in the first slot and they are using the channel select bits (X15, X16). Can you zip the entire project and post it?

Oh, and please state the actual 405 CPU type, the analog card type and the slot it is installed in.

The automationdirect.com website has manuals for the analog modules available for download.

edit:
Link:http://www.automationdirect.com/static/manuals/d4anlg/d4anlg.pdf

Bernie,

Attached is the .prj file. The CPU type is 450. Also, I see the LDF block, but according to the help file the LDF block:

The Load Formatted instruction loads 1-32 consecutive bits from discrete memory locations into the accumulator. The instruction requires a starting location (Aaaa) and the number of bits (Kbbb) to be loaded. Unused accumulator bit locations are set to zero.

Click to expand...

What I don't understand is how discrete bits in the accumulator translate into raw analog values. The rack configuration and analog card type I don't know yet, because the PLC is located at a remote location. I'm working on getting that information now.

1. Numeric values in a computer (a PLC is a computer), such as an analog measurement, is made up of a series of bits. The least significant bit has a value of '1'. The next higher bit has a value of '2'. Each higher bit has twice the value of the lower bit. Each '1' bit takes on the value of its position with the final value being the sum of each position containing a '1'.

2. In rung 67 the LDF command is loading the 12 bits from X0 up to X13 (this is octal numbering), from the analog unit in slot 1. The 12 bits can represent a 12 bit number (from 0 - 4095). By this action the representation of an analog value has been loaded into the accumulator.

3. The analog card is indicating (by use of X14 thru X17) which of 4 analog channels on the card this particular number represents. The channel being read will probably shift each pass through the program. More detail would involve knowing the precise card being used and reading its manual. The following 4 rungs use the indicating X input to direct the reading into one of 4 destinations.

4. Subsequent rungs are doing the same for other analog cards on the rack (there appears to be 5 four input cards.)

5. After that further application specific processing begins.

If you wish further analysis more data is required. I hope this helps.

Bernie,

Thank you for your detailed and informative reply. It clears up my confusion greatly. I finally got a rack configuration and will read through the appropriate manual. Again, thank you for taking time to help me.

NatureUhFindsAWay said:

2) Is the PLC also smart enough to clear the accumulator to make way for the next input?

Click to expand...

Normally, loading the accumulator, or any other register for that matter, will merely overwrite any previous value held therein.

Glad to help.