While the worry of a conflict when such pin turns to output (and to zero) is slightly adequate, you still should have full control on your inputs and outputs. But the following is a bit of exagerration.

1. No internal pullups on port 0. No such worry there.
2. A quick glance into a typical '51 datasheet (AT89S5x this time):

I_IL Logical 0 Input Current (Ports 1,2,3) V_IN = 0.45V max. -50 µA

OK so this means that the internal pullup is equivalent to 100kOhm or so. To overcome this, let's assume 0.45V is a pretty good zero, hence 9kOhm pulldown is enough. OK, let's make it 4k7, it is my favourite off-the-drawer resistor. It means, you need to "source" roughly 1mA with the optocoupler's transistor (and it does not matter at all if it is NPN or PNP, it has a floating base and it has no way to find out, how it is connected!!!). With a standard PC816 with CTR_min=50% this means 2mA at the LED, no extremity I think.

While there is probably no justification to use such circuit with a '51s input (and there is the risk of turning it to output and to zero at a wrong time), there are situations where such circuits can, and should be used. One typical is an optically decoupled RS485, where the RS485 side has an independent power source; with a standard 75176-and-alike you need to keep your DE at zero even if the '51 has no power.


I found that it is a quite common practice here to say "this is bad", rather than "this is uncommon and use it with care only if really needed, because of this and this".

Jan Waclawek