Re: The 250 byte packet viz. ESTOP

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Wayne Weedon <wayne@...>
wrote:

>
> John Dammeyer wrote:
>
> >Hi Mark,
> >
> >If the USB dongle is receiving 250 bytes every 10mS and checks

the ESTOP and

> >limit switches every 40uS then even if there wasn't a immediate

message to

> >the PC via USB to tell it that an ESTOP occurred worst case would

still be

> >only 10mS.
> >
> >
> E-stop signal to the cpu should only be a signal to say E-stop has
> occured. Normally E-stops are hard wired externally, and any

processor

> will have nothing to do with it.
>
> Drive(s) shutdown is all handled by the estop relay on all my

retrofits,

> but then at least here in the UK it's a legal requirement to do it

this

> way. I'd expect it's much the same elsewhere.
>

<snip sig>

I handle E-stops similarly, although in an unrelated field (chemical
processing). First, I'm just a lurker here, interested in someday
building a hobby router. On my lab rigs (automated chemical
processing miniplants), I generally use a main power relay, with all
E-stop activity controlling the relay. Opening the main relay
usually stops all pumps, motors, and heaters, but not always. It
depends on the process.

Things that need to be are hardwired in series (usually E-stop
buttons) and one relay in the series is controlled by the control
program. This is because the control station is or can be remote,
and other conditions (usually temp and pressure) may require an E-
stop. The power to the 24VDC power supply comes off ahead of the
main relay because in this situation I need to know what's going on
in the process. Chemical processes don't necessarily stop if you
cut power.

In any case, I try to anticipate and test for anything that might
contribute to an unsafe condition, and design accordingly. This
could include dumping or quenching reactants. An extreme and huge
case is light hydrocarbon plants, which must have automatic and
redundant flaring of all products on loss of refrigeration
compressors or product pump power. Otherwise, you have the world's
biggest fuel-air bomb.

At least in the CNC world, when you cut the power everything stops,
so that would be the safest condition.

Regards,

Pete Keillor