qeustion bout a-d and d-a

My question is how does analog to digital and digital to analog convertion takes place within the analog input and output units on the omron plcs. I understand so far that , just say for the analog input unit that it recives a analog voltage then converts it to a 4 digit hex, NOW it that the end of the story or is their more to it than this?

I don't know about Omron, but generally the process goes like this:

(Input)
Signal is received (4-20ma)
It is converted to a 1 to 5V signal.
This is then run through a AtoD converter. The most popular seem to be 12 bit, but you can get anything from 8 to 16 bit depending on the manufacturer. I will assume 12 bit for the rest of this analysis.
Once the conversion is complete, the value is placed in a register. This is generally a 16 bit register, giving you a value from 0000 to 0FFF (Hex).
This value is then used as the PLC process variable.

This is the basic process that all PLCs use, although each step can have its unique variations depending on the whims of the manufacturer and customer demands.

One popular variation is to convert the process variable into a floating point value. In that case, it is scaled between a minimum and maximum value. Example, you have a pressure input from 1 to 6 bar, with 1 bar = 4ma and 6 bar = 20ma.

You setup the scaling in your PLC to 0000 = 1 bar and 0FFF = 6 bar, and the conversion does the rest.

Hopefully this will give you a bit of a better insite into how it works.

You would have to check the manual for the card as they do vary (Omron I mean). The manual will tell you the maximum count each card makes available and there are examples in the respective manuals for scaling etc. Ther are also other ways to scale the inputs. The outputs drive directly through a D-A converter. You just have to place the appropriate value in the channel allocated to the output. D-A is dead easy.

Here is my understanding of the A-D cards.

A-D cards are somewhat different. These cards have several input options. They can be 4-20ma, 1-5v, 0-10v, -10 - +10v etc. Once again it depends on the card. For example, most cards have 3 connection points plus the screen. The main 2 points are 0V and +V. The 3rd point for each input pulls in a resistor for 4-20ma inputs. Basically, the +V point is jumpered to the +ma connection point.

This input then goes through the A-D converter and into the digital processing section of the card.

When setting up the card in the SIOU unit setup in CX-Programmer, you can select whether to enable or disable the input/output. You then select the input type ie 4-20ma, 0-10v etc. You can enable or disable each point individually. You can have different types of input on each point ie the card does not have to be all 4-20ma. You also can set up an average read function eg 4 = the average of 16 reads before placing the data into the input channel. The reads are buffered internally on the card and the average calculation also takes place there. For 4-20ma and 1-5v inputs you can then setup input failure detection. If my memory serves me correctly this is calculated at less than 3.8ma for a 4-20ma input. If this value is reached, the analogue card will turn on a bit in the PLC to be used as an alarm. An alarm light also is shown on the analogue card.

Hope this helps.