I'm trying to interface 74HC123 monostable with 6N137 optocoupler. Signal from optocoupler will turn on MOSFET driver. Monostable outputs correct signal when optocoupler is not connected - 200 ns pulse. When I connect optocoupler with monostable the voltage on monostable pin 4 is pulled down. I thought that it was due to low output current of monostable, but this is ok according to datasheet. Maximal monostable output current is 25 mA, LED in optocoupler will take around 10 mA.
Monostable is activated by pushbutton. I need positive pulse on optocoupler output. IC's are setted according to highlighted fields in truth tables below. Monostable output pulse width is setted byt C3 and R2.
Osciloscope images of button click, and output monostable pulse with not connected optocoupler are below.
Monostable datasheet: http://www.ti.com/lit/ds/symlink/cd74hc123.pdf
Optocouplet datasheet: http://www.mouser.com/ds/2/239/S_110_6N137%2020140411-596065.pdf
C1 and C4 are decoupling capacitors.
C1, C4 = 100 nF, C2 = 82 nF, C3 = 100 pF
R1 = 8k2, R2 = 2k, R3 = 330, R4 =2k2
74HC123 monostable truth table:
6N137 optocoupler truth table:
Any idea what may be wrong? Is this correct way for interfacing this two parts ( high output idle state of monostable )?