watchdog timer error

[Guest aud]

We have instaled an LVDT and load cell in a press machine an I need to capture readings to see the comportment Force Vs Displacement. So that am doing is next: once linearized (LVDT & Load Cell) by mean SCP (Micrologix 1500 LSP proccessor) 1.-Start Cycle and Press down 2.-At this time an 1769-IF4 (ch0)read meassurements from the lvdt before an during the press do contact with the components to assembly and program start to save 240 data in a N11:0 to N11:239. 3.-In the same manner once the press do contact with the componnents the load cell send the readings al 1769-IF4 (Ch1). and 240 data are seved at N12:0 to N12:39. so am using an timer at PRESET= 0.001 as oscillator. and increment an Reg N7:0 that is used as index pionter to direct each one 240 data block (N11:[N7:0] and N12:[N7:0]) during each true to false transition. next i send this registers to excel with RSLINX|MY_TOPIC!'N11:0'....N11:239' and that I hope to see is a ramp in the force graph but isn't hapen because only see a shoot in force at time that components are assembled and no Force ramp. The problem is that at timer 0.001 the capture is very low am try to use an LBL and JMP instructions to dedicate the program to capture data but the proccessor goes to FAUL mode an apeare an error code 22h that indicates an error with the watch dog timer. how can i reset the watchdog before the error be sends. Thanks.

[Ken Moore 23]

You can change the watch dog timer value in the controller status file, the number shown is times 10 so a value of 20 would have a watchdog value of 200ms.

[Guest aud]

Tanks, acctually i have this value set at 200 and even still goes to fault

[Ken Roach 194]

I don't think you are approaching this correctly. Let's check your assumptions and your configuration. Have you considered how fast this module can convert analog signals to digital format ? Analog modules are generally designed for precision, not speed. Consider how a timer is evaluated by the controller. Timer accumulated values still have to be evaluated by the controller scan. If the timer is 1 millisecond, but the controller only scans every 5 milliseconds, how fast do you think the timer is actually acting ? You have a watchdog fault because you have used JMP and LBL to form an infinite loop, or at least one too long for the MicroLogix to ignore. I suspect it is infinite because increasing the watchdog to 200 x 20 ms = 4 seconds didn't solve the problem. The default filter value on the 1794-IF4 is 60 Hz, so you would not get more than 60 different values per second even if you ran the MicroLogix fast enough to sample 1000 times. The fastest filter the IF4 is capable of running is 500 Hz. I would recommend either the Selectable Timed Interrupt in the SLC controller for this function, or just scanning as fast as possible and indexing your data storage every single scan.