Thread: Ortonview PCB
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Old 29th Jun 2021, 6:20 pm   #19
Slothie
Octode
 
Join Date: Apr 2018
Location: Newbury, Berkshire, UK.
Posts: 1,287
Default Re: Ortonview PCB

Quote:
Originally Posted by SiriusHardware View Post
Seriously though, this is why adding these little safety catches - a 2p resistor (which might potentially save an old bit of silicon costing 100 times as much) is worthwhile. There is an ever decreasing supply of these expensive old devices out in the wild.

Maybe the pulldown switches on the page select and mode select inputs should also have low value series resistors in case 8154 outputs are ever accidentally connected to them when the switches are in the short-to-ground position.
How low a resistance will protect the 8154? The data sheet I have for the 8154 doesn't specify a maximum output current, and I'd worry if the resistance was too high then the 8154 wouldn't be able to pull down the option pins enough to register as a '0' on the PIC.. Even a 500 ohm resistor would only limit current to 10mA but would require the 8154 to pull the pin down to less than ~0.5v.
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