i was in your same delima at my last job.
this is what i preached and still do andd is my 2 cents.
this is a lengthy post, but it explains what you need.
you asked the question what is the difference between online data and off ine.
online data is the current values in the plc program as it runs, timers, counters, integers, real, control registers.
off line data is what is in the plc rogram when you saved the program to the pc.
RULE #1
the program in the plc is the current program and it is the master copy !
all plc programs stored on network drives, pc's, memory sticks are backup copies only
and must be considered incorrect. the only time backup copies of the program are to be considered the masters, is when the current program has gone nuts due to bubba making changes and can't remember what he did!!
when going online, make a backup copy of your program first, then upload the program if required.
then when you can get online with no issues, you can edit the program and save it. backups can be your friend.
Rule #2, when in doubt, refer to rule #1
Step 1. have a folder on the network created that you, your manager, and IT can read write to, NO ONE ELSE!
this is the master copy where the plc programs will be stored.
have a second folder created that everyone can access with read / write capabilities for maintenance.
step 2. programming naming scheme.
we had over 50 buildings and various areas in each building.
separate the plc programs into buildings, departments.
name the plc program based on the machine and then add the date, for example extruder23-11-14-18.
the machine name is extruder23 and the date is 11-14-18, this is when the program was saved / modified.
when there is an issue, maintenance copies the program to their laptop or opens the file in the maintenance folder and uses it.
when a minor change is made (bad input / output) document the input / output is bad and save the program under the same name.
when logic is changed, say in 3 weeks, save the program as extruder23-12-5-18 and document the change.
test the program change for a week or two. if the issue is solved, copy the program into the master folder.
Step 3.
discuss this plan with your boss first, get his input.
then discuss with engineering, maintenance managers.
then discuss with the maintenance personnel who can make changes.
each programming laptop has a user name and password that only they know, this helps keep bubba outof the laptop. they DO NOT give the password to anyone! change the password every 90 days or so.
programs are to be stored on the maintenance folder ONLY!
no jump drives, floppies, or personal files.
when programs are modified, have them notify you and your boss of the change, the program name and reason for the change.
have them save the new program back to the folder.
you then get the notice, see what was done, and tell them to erase the program from the pc.
explain to them that there will be consequences for failing to follow the rules.
written warnings, laptop confiscated and no more programming capabilities.
it will be hard at first, but eventually you will separate bubba from the capable people.
i had to goto every machine and get a good copy, wasn't easy. there was plc programs, panelview programs, rsview programs, speciality displays and so on.
at my current location, we have read only allen bradley software for most maintenance people.
they can go online, toggle and force i/o only.
there are only 2 wonder ware programming stations controlled by the IT department (we are the programmers and all are engineers). there are 2 maintenance laptops for slc500 and plc5. they use the read only version of logix 5000 when required. there are 2 plc programmers and a manager we trust.
again, this is what i had to do and is my opinion
hope this helps,
james