If you have a spare Arduino Mega, you can set a switch in the firmware (Config.h FAKE_SERVO) that let’s you play without having chains and motors attached 
, one of my favorite ‘hidden options’!
The firmware sends position and position error information 5 or 6 times a second, so prepare to ignore or filter out a lot of chatter in the channel. Watch those values to see your gcodes have effect - they are used by GC to put position and speed on the screen. Send “$$\r\n” to see a dump of internal settings from the firmware.
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