PLC Data Logging System: Some basic questions

Question

I am currently trying to work with PLC. I am using kepware data logger to collect the PLC log data. The output is like below:

 Time Stamp                      Signal                          Signal O/P
20130407104040.2    Channel2.Device1.Group1-RBT1_Y_WORK_COMP_RST     1
20130407104043.1    Channel2.Device1.Group1-RBT2_Y_WORK_COMP_RST     0
........................

I have few questions:

1) What does that 'Channel', 'Device', 'Group', 'RBT1_Y_WORK_COMP_RST' means ? - What I have got from the PLC class presentation is that: RBT1 (which refers a robot) is a machine and 'Y_WORK_COMP_RST' is it's one signal and 1/0 is the signal state at particular timestamp (like 20130407104040.2). But, I could not get from log data file what is: 'Channel', 'Device1' and 'Group1' means ?

2) I learned from classes that 'PLC is a hard real time system'. However, from the log data file I am seeing that: the cycle time differs often. I mean some time it takes (say) 5 seconds, sometime 7 seconds. Why that ?

3) Does this log data taken by kepware is the actual machine output ? Or taken from the PLC program ?

NB: I am very new in this field and taken very few classes. So, may be my questions are stupid. Please help me by giving some basic not so technical answer.

Answer 1

1) Channel2.Device1.Group1... is the path where your KEPware data logger could find your RBT1. If you add another device with another technology you should get something like : Channel3.Device1.Group1.... This is totally internal to KEPware data logger and have nothing to do with your PLC. What interest you is the last part of the path : RBT1_Y_WORK_COMP_RST

2) Are your PLC and the PC running the KEPware data logger time synchronized ?

3) You are connected to a PLC so the KEPware data logger take data from it, then your PLC has to be setup to collect the output of your machine if you want to do so.

Answer 2

1) The channel is the type of communication, it may be several communication protocol, like modbus or devicenet or whatever kepware supports.

The device is the device Kepware communicate with and the group is just some way to sort your items items will refer to your plc address and let you name the item as you wish. This way you got an easy to read alias of your address.

2) Hard real time systems means the PLC must react to its input change within a certain amount of time (Ref: Wikipedia) Most of the time PLC are programmed in Ladder, Ladder is sequential and depending of the step the program takes it maybe longer or shorter. Also the timestamp comes from Kepware, not the PLC, so it depends on kepware scan time as well.

3)Kepware connects to the PLC and request PLC address with the output status.