About homing and switches...

[john swift]

As usual
the devil is going to be in the detail
when looking for the solution to problems connecting limit switches to breakout boards
for instance the LJ12A3_4_Z_BX proximity switch does not have the simple open collector NPN transistor output you may expect going by some adverts and data sheets
It has a hidden 10K pull up resistor connecting the output to the positive supply !


with ref to the diagram in post 24

adding the diodes works with the usual TTL inputs found on most BOB's
that requires the switch to either ground the input or go open circuit

some Chinese BOB's have opto isolated inputs with a PC817 input LED and current limiting resistor connected to +10V

the Geckodrive G540 has its opto isolator connected to +12V and has a 2K2 current limiting resistor


so you don't always see 5V across the BOB's open circuit input and ground terminals !

BOB's like CNC4PC's C31 need a positive 24V input
so if you are using proximity switches you need one with a PNP output
to pull the BOB's input up to the positive supply

many Chinese BOB inputs connect to either a 74HC14 or 72HC245 logic
IC input
(see the upper left hand corner and centre of this diagram for the Chinese BOB input
Lower left hand corner for the C31 )



( I prefer N/C switches because a broken wire will be trip the axis limit
with N/O switches a broken wire will not be detected )


John