2023-07-30, 12:02 AM
For those playing along at home:
I ditched the opto-coupler and took some inspiration from a supposedly "official" Seatalk1 circuit:
https://www.electro-tech-online.com/thre...ce.130122/
The Seatalk1 circuit on the MacArthur HAT v1.1 looks like this:
That worked fine on my and Sailoog's bench, but failed again in the field. Some probing by Dylan revealed, that the issue was transistor Q5 not fully turning off when SEATALK_DATA goes low. This is fixed by replacing resistor R21 with 20K.
This lowers the (theoretical) voltage level on the base of Q5 from 0.5V to 0.3V, giving more headroom to the transistor's Vbe(on) which is specified as 0.58-0.7V.
I ditched the opto-coupler and took some inspiration from a supposedly "official" Seatalk1 circuit:
https://www.electro-tech-online.com/thre...ce.130122/
The Seatalk1 circuit on the MacArthur HAT v1.1 looks like this:
That worked fine on my and Sailoog's bench, but failed again in the field. Some probing by Dylan revealed, that the issue was transistor Q5 not fully turning off when SEATALK_DATA goes low. This is fixed by replacing resistor R21 with 20K.
This lowers the (theoretical) voltage level on the base of Q5 from 0.5V to 0.3V, giving more headroom to the transistor's Vbe(on) which is specified as 0.58-0.7V.