Created Date: July 12,2002

Created by: BradB

I have a high speed orienting application which requires an adjustable timer value. What I am looking for is an HMI with an extremely minimal impact on PLC scan times. What is the difference between the DV-1000 and EZ Text panels in this repect?

I have been told that with the DV-1000, the PLC is the master and the display is the slave, whereas the opposite is true with the EZ Text. And when the display is the master, the write time to the PLC is quite negligible. Is this so? What kind of scan-time delays could be expected for a single V-memory location write? (I will probably be using a DL-05)

Thank you for you input.

Created Date: July 15,2002

Created by: franji1

Brad,

I have never seen communication adversely affect the PLC scan time. The PLC actually has "time slices " in the firmware for these tasks, which is negligible compared to the user scan time. If a task takes longer than the designated time alotted, it saves where it is and continues on the next PLC scan. An example of this would be "syntax check ", which takes some time to do, but occurs across multiple PLC scans.

However, for basic HMI operations, such as reads and writes, the time will be negligible.

However, if you have 32 ECOM modules on a 450 using all 4 COMM ports thrashing on the PLC, you would see some affect on the PLC scan time, but it would probably be a few milliseconds per scan.

However, re-reading your post, I'm wondering if you mean "latency ". That is, how long does it take from when I say "write this value " does the write actually occur. This has no affect on PLC scan time, but has great affect on PROCESS scan time (there is a HUGE difference between the two).

If this is the case, you may need to go Ethernet (time on the wire greatly affects latency) - which is not yet available for the 05. A 240 or 250 with an ECOM module, along with an EZ Text panel with an EZ Ethernet Card will be 50x faster than serial.

Please let us know what you mean by "minimal impact " - if you mean PLC scan time impact or the latency impact.

Created Date: July 16,2002

Created by: BradB

Thank you for your reponse.

I do mean PLC scan time. The communications latency is not really a problem. I had run into this issue on a DL-205/DV-1000 project once. The PLC had to write quite a bit to the display, and the PLC would randomly miss some of the more time critical inputs because the scan time got so long. This was resolved after optimizing my code. (I used 1-shots to write to the display wherever possible.)

This new project is much more time critical. A clutch/brake is used to stop a high-speed rotating part a user-specified length of time after an eye sees a registration mark. In this application, a scan-time "hiccup " of a few milliseconds could translate into a few degrees of error in rotation. That is why I need a very smooth, short, and predictable scan time. Perhaps this funtion would be better served by some kind of hard-wired system external to the PLC?

Thank you for any further advice or input you might have.

Created Date: July 16,2002

Created by: franji1

If you are looking for a discrete input to turn on or off a discrete output, you could possibly use an interrupt routine with immediate output instructions.

The interrupt routine would interrupt whatever the PLC was doing at that moment, execute the ladder logic in that routine, then continue where it left off. By using "immediate " instructions, you could turn a discrete output on or off "immediately ". See INT and STRI/ORI/ANDI and OUTI instructions.

I believe the 05 support one hardware interrupt, High Speed Input Mode 40. See page 3-44 in chapter 3 of the 05 online manual , along with page 5-26 in chapter 5 for information on Immediate instructions and page 5-103 in chapter 5 for information on Interrupt instructions .

Created Date: July 16,2002

Created by: KPrice

BradB,

We have had a similar project and I can suggest the steps we have used to reduce reaction time. Try using stage programming with only the timer/clutch/brake output stage active (along with any other required stage, such as alarm switch or operator control button monitoring stages). The PLC only scans active stages. An Out Immediate instruction will output the on/off status to the module output point (and image register) at the time the instruction is executed.

Created Date: July 16,2002

Created by: BradB

The interrupts, immediate I/O, and isolated stages all sound like worthwhile ideas to check out. The interrupts sound especially promising. I do have one more question, though.

Does the interrupt function allow you to incorporate a timer, for an immediate input, a precise delay, and an immediate output? (The delay would be fairly short - less than 100 ms.) Thanks again.