Hey, I had no idea this $SLP command existed, I really need to re-read that GRBL doc from cover to cover one day.
I double checked the CM keyboard shortcuts and there is apparently (unsurprisingly) no shortcut for Ctrl-X. The only way with CM would be to go to the MDI and try to send it from there, so…out of curiosity I tried (on my Shapeoko),
- sending “^X” does nothing (CM probably filters that)
- adding a / as mentioned in the MDI window to send directly to GRBL does send ^X, but produces this:
(2128): <- [GC:G0 G54 G17 G21 G90 G94 M5 M9 M56 T0 F0 S0]
(2127): -> $G
(2126): -> gc_parser_state
(2125): <- ok
(2124): <- [PRB:0.000,0.000,0.000:0]
(2123): <- [TLO:0.000]
(2122): <- [G92:0.000,0.000,0.000]
(2121): <- [G30:0.000,0.000,0.000]
(2120): <- [G28:-4.996,-4.996,-5.000]
(2119): <- [G59:0.000,0.000,0.000]
(2118): <- [G58:0.000,0.000,0.000]
(2117): <- [G57:0.000,0.000,0.000]
(2116): <- [G56:0.000,0.000,0.000]
(2115): <- [G55:0.000,0.000,0.000]
(2114): <- [G54:0.000,0.000,0.000]
(2113): -> $#
(2112): -> gc_get_offsets
(2111): <- <Idle|MPos:0.000,0.000,0.000|Bf:14,128|FS:0,0>
(2110): -> gc_sync
(2109): <- error:1
(2108): -> ^X
(2107): -> gc_not_motion
I don’t know, I may not be sending the right ASCII characters, tried with both AltGr+9 to produce the “^” char, and with the actual “^” key, same result.
Anyway, I suspect that Carbide will not like to add an auto-SLP after a given timeout, there is WAY too much risk to mess a multi-hour job for someone 
(imagine being 10h into your 14h cut, and CM decides to go to sleep…)
Honestly, the simplest solution seems to be to rewire the machine power supply to a different plug/switch than your enclosure lights (I know I did)
Oh, I almost forgot : “CNCjs lets you send soft resets”