The weakest link

As a newbie to Linux and LinuxCNC, why does Latency matter less for a servo system with feedback?

And following on from this, Is there any major difference in the requirement of the PC depending on what the application is?

For example I see a lot of people on these DIY CNC forums are running routers with stepper motors and linear ways, on which the machining rate, and therefore feed is a lot faster, which I presume means more calculations as more moves have to be done in a short time

than compared to

A traditional solid way metal milling machine which is going to be operated at a much slower machining rate, and therefore lower feedrate, using servo drives with feedback to LinuxCNC?

It would certainly makes things easier if some computer suppliers could put together a package that was LinuxCNC friendly, or as was mentioned a BOM that will give you a decent PC using readily available and current parts

As a newbie to Linux and LinuxCNC, why does Latency matter less for a servo system with feedback?

Very good latency (say less than 30 usec or so) matters mainly for software step generation systems (typically using the PC parallel port) since these systems need to generate regular pulse streams at 5 KHz to maybe 50 KHz. Large latency spikes on a software step generation system interfere with this pulse stream generation and may cause step motor stalls.

Latency is much less important on servo systems or step/dir systems that have hardware step generation. Servo systems and hardware step/dir systems typically only have a servo thread running on the PC at 1 KHz (1 ms period). Jitter in the servo thread only causes second order errors (mainly that the position sample time has jitter which will look like position jitter (proportional to velocity)
to the PID control loop. 100 uSec or more jitter is tolerable on these systems. This means almost _any_ PC can be used for servo
or hardware stepgen systems.

If even better jitter tolerance is needed, Mesa hardware has an optional DPLL that allows re-timing of encoder and stepgen position sampling. This can be used to make a system that will tolerate even 500 usec of jitter in a 1000 usec servo thread.

And following on from this, Is there any major difference in the requirement of the PC depending on what the application is?

For example I see a lot of people on these DIY CNC forums are running routers with stepper motors and linear ways, on which the machining rate, and therefore feed is a lot faster, which I presume means more calculations as more moves have to be done in a short time

than compared to

A traditional solid way metal milling machine which is going to be operated at a much slower machining rate, and therefore lower feedrate, using servo drives with feedback to LinuxCNC?

It would certainly makes things easier if some computer suppliers could put together a package that was LinuxCNC friendly, or as was mentioned a BOM that will give you a decent PC using readily available and current parts

Actually I don't think there is a big difference between router and machine tool requirements as machine tools are are slower but typically require better accuracy so may process as many line segments per second as a fast router.

For a servo or hardware step generation system almost any PC will be satisfactory,
if you are trying to get 50 KHz pulse streams out of a parallel port, it will be nearly impossible
to find a PC that will work well.