Block Transfers in PLC-5 whether local/remote work in an asynchronous fashion. This implies the data as well as the status bits (Done, Enable, Start, etc) are asynchronous to the PLC-5 program and I/O scan. Recommended thing by many sources is to copy the data and the status bits to another set of registers (i.e. buffer area).

1rst, where in the program scan do I copy this data to the so called buffer area?

My problem is: I need to know when fresh data has
arrived from speciality I/O which includes VHSCE, motion controllers (QB), Analog input cards, etc. I am running a motion application and that application requires continous motion.

My other major problem is I need to know when fresh data comes in perfectly (very critical that I can not have old data in there since I am performing extensive math in the PLC).-I can not just use the done bit because it remains on until enable bit goes on again and I really need the scan time so I prefer not to slow it done too too much by using other items like one-shots but if only solution, please let me know.

From one integrator to the other or user, I would greatly appreciate it if you can shed some light on this matter.

Thank you so much.
Kind Regards,
[email protected]

 

> Block Transfers in PLC-5 whether local/remote work in an asynchronous fashion. This implies the data as well as the status bits (Done, Enable, Start, etc) are asynchronous to the PLC-5 program and I/O scan. Recommended thing by many sources is to copy the data and the status bits to another set of registers (i.e. buffer area).

*****I will trust you on this one.*****

> 1rst, where in the program scan do I copy this data to the so called buffer area?

******It wouldn't matter since everything gets set at end of the scan...unless you are using immediate input type blocks. The status may? be asynchronous but the 'buffer' registers would be normal PLC scanned registers.********

> My problem is: I need to know when fresh data has
> arrived from speciality I/O which includes VHSCE, motion controllers (QB), Analog input cards, etc. I am running a motion application and that application requires continous motion.>

*****I would try a one shot (ONS) on the done bit.
Would it do anything to put the BTW/BTR on a timer done bit? I would guess this might solve some problems, but create another (speed)****

Just my two cents worth.

 

I would think that most times you would BT quite often and repetatively. The standard way is to start a new BT when one is done. Also RSLogix5 can setup the BTW and BTR required for most modules, except the SDN will require more work. All the module manuals will give examples of the BTR and BTW instructions. Also using the BT instructions will require allocating a group of integer files for each instruction.

Cheers.

 

I don't know if it'll work for you but, take a look at the DTR instruction. When your data changes you get a one-scan-only logic true condition for your rung. Good Luck.

DP

 

Which is why you do NOT use a PLC for motion control. You're going WAY out of your way to provide determinism, I guess - in ladder logic. I assure you, the time you waste trying to get this to work and then the time to attempt to tune the motion loop will be WAY more than even an expensive SERCOS based motion controller. Bring any signals critical to the motion directly into the motion controller. Then, if your PLC needs any of this information, use any number of methods to get it over there.

Give it up now, while it's still early. Do a search for motion controllers with I/O or contact me direct and I'll fill you in on all kinds of options.

Robert B. Trask, PE
MindSpan eSciences
Wilmington, NC USA
[email protected]