19kHz free-runing clock on MicroLogix 1xxx

[drbitboy]

Does anyone know the history of the A-B 10kHz free-running clock?


It seems to only exist in the MicroLogix 1xxx line: S:4 is the 16-bit clock itself; S:35 is the duration of the last scan in 100us steps. Why is it not available in all PLCs?


The free-running clock is very useful: S:35 can be used to implement a repeating timer functionality that triggers as reliably as possible in real-time. Of course the accuracy of any one trigger is no better one scan time plus any drift in the clock's base oscillator, but the mean period will be precise wrt the target. A TON will always run slower than what is specified in the .PREset.

 

[gclshortt]

http://www.plctalk.net/qanda/showthread.php?t=99188

" all MicroLogix and SLC family controllers have a free-running clock based on the controller's crystal oscillator that is stored in the Status data file in word S:4.

The individual bits of that clock can be used as 50% duty cycle flashers.

Because they're based on the crystal and not on the real-time clock, they don't have an exact 1 Hz period. Bit S:4.07 will be on for 1.28 seconds and off for 1.28 seconds.

Don't try to use the Real Time Clock (RTC) data file for a 1 Hz flasher; the Seconds value is only updated every two seconds so you never see any odd numbers."

The above may help.
Regards,

 

[drbitboy]

http://www.plctalk.net/qanda/showthread.php?t=99188

" all MicroLogix and SLC family controllers have a free-running clock based on the controller's crystal oscillator that is stored in the Status data file in word S:4...

Garry, Thanks, I was looking at the PLC-5 manual and it did not have the free-running clock, and I had a senior moment and confused PLC with SLC - d'Oh. So, SLCs are the ancestors of MicroLogix 1xxx line then?


But this is not available in other A-B hardware, e.g. ControlLogix, only the 1Hz RTC accessible via the GSV instruction?


Huh, I see MicroLogix 8xx has a 100kHz clock and an RHC (Read High-speed Clock) instruction, so that is nice.



Sorry about the typo in the thread title; it should be 10kHz, not 19kHz.



Does anyone know the history though? It seems like such a basic function, and there is almost certainly a multi-MHz oscillator somewhere, so why not make it, or some, divide-by-eight or -sixteen version of it available to the programmer?

 

[the_msp]

So, SLCs are the ancestors of MicroLogix 1xxx line then?

The first system I worked on was replacing a SLC 150 (with the 150-E (Expansion)). It was a brick style SLC programmed in DOS. The 150 was placed on the market before I was born.

The chassis style SLC started with 5/01, which was to be a Small Logic Controller (SLC) version of the PLC5 system - smaller chassis, tighter constraints on maximum IO.

Within the same project I also upgraded a 5/02 to a 5/03 and replaced the DTAM (Data Table Access Module (HMI)).

Does anyone know the history though?

@ Ken Roach