485 input attenuation circuit protection modification

Mark Vincent

For those that have a 485, look at the S/N to see if it is in the lower range. At one point it was changed to protect the round IC from pluses from the relay coil when switching from 50 ohm to 1meg input. You can modify the circuit by adding the parts of the higher S/N to the lower to protect the IC. I did this in mine. The schematic will show both versions. The NPN transistor that is added is a 2N3904. A 2N2222/A or other will work.

There is also a 12V axial tantalum that is for the 9V filtering on each input. I removed the tantalums and put in a ULD 16V to prevent those tantalums from shorting. There is a 13V circuit that has 15V tantalums that will short. I used 47mfd 25V ULD types in this circuit.