Classic Ladder and thread timing

I am retrofitting a circa1995 Howa VMC from a Meldas 520 controller to LinuxCNC. As of today, all the original IO (~ 40 inputs & 40 outputs) are operational on 2 MESA 6i24 cards, along with 7 Mitsubishi servos on one of Dmitry Yurtaev's YDSMP2 cards.  This is all on a servo-thread period of 888888ns, as required by the Mitsu serial protocol.  I have full ladder documents for the  machine, and was looking to translate it into ClassicLadder. LinuxCNC docs mention ClassicLadder can't update rungs faster than 1ms.  A couple of questions:

• Should I create a separate thread that is a multiple of the 888888ns servo-thread to run the ladder (1.7777...ms)?
• Is there a particular reason for the cap of the ClassicLadder refresh rate at 1ms, other than being set in SRC?

Thank you for the info. This is what I gathered from the LinuxCNC manual:

"13.4.2 Realtime Module
Loading the Classic Ladder real time module (classicladder_rt) is possible from a HAL file, or directly using a halcmd instruction.
The first line loads real time the Classic Ladder module. The second line adds the function classicladder.0.refresh to the servo
thread. This line makes Classic Ladder update at the servo thread rate.
loadrt classicladder_rt
addf classicladder.0.refresh servo-thread
The speed of the thread that Classic Ladder is running in directly affects the responsiveness to inputs and outputs. If you can turn
a switch on and off faster than Classic Ladder can notice it then you may need to speed up the thread. The fastest that Classic
Ladder can update the rungs is one millisecond. You can put it in a faster thread but it will not update any faster. If you put it in
a slower than one millisecond thread then Classic Ladder will update the rungs slower. The current scan time will be displayed
on the section display, it is rounded to microseconds. If the scan time is longer than one millisecond you may want to shorten the
ladder or put it in a slower thread."

I downloaded the linuxcnc_master file and glanced at the classicladder source files. A few of the config files having timing defs based on 1, 10, 100 ms intervals. I'm thinking that may be for convenience in setup and Modbus programming when the code was first written, and nothing 'sacrosanct' about the interval. Could require a lot of checking and recompiles to change it.

I have a hunch once I start translating pages of ladder and get more conversant in it, I may wish I'd just gone after this differently. But you have to figure the Japanese engineers that designed this machine back in 1995 knew what they were doing. The quality of the hardware is incredible.

i want to clarify, maybe it will help you.
though the servo thread is running at 0.88ms, communication with servo amps happens every 4th cycle, i.e. 3.55ms period. it is so because the command packet includes 32-bit target position plus four 16-bit offsets at 1/4th period. i think MDS-A-SVJs ignore those, but MDS-x-V being "advanced" amps may use it to bring the effective cycle period to 0.88ms.
so i think if you run the classic ladder in servo thread - that should be enough even if it somehow will slow down to 1ms...

Thank you for the clarification on your board timing, Dmitry. And thank you for clarification on the ClassicLadder documentation, Tom.

Cl is limited to 1ms on the source.
From memory I couldn't get it to run efficiently at a faster rate.
You could change the source to try. I'll have to look for the file name when I get home.

github.com/LinuxCNC/linuxcnc/blob/2.9/sr...er/module_hal.c#L119