I would recommend that you place a pullup resistor from the opto-coupler collector to the Vcc. This resistor will ensure that the I/O pin stays high when the optocoupler is OFF.

Some designers may try to promote the idea that the internal pullup resistor in the 8051 quasi-bidirectional pin architecture is enough to ensure the high state of the OFF opto-coupler output but I have two reasons for recommending otherwise. First off there is the issue of noise coupling and the susceptibility to it. A 10K pullup will lower the impedance of the high input to the 8051 such that there is much less likelyhood that stray coupling can move the signal level around. Secondly, opto-couplers are notorously slow, in particular on the output signal rise time as the transistor has gone OFF. The pullup resistor will provide a faster rise time and its value can be selected to suit your design requirement.

Note that it is my general practice to not rely on the weak pullups of the 8051 port pins to provide high levels on the pins as inputs whenever there is any length of copper signal trace attached to the I/O pins. I have less concern when the pins are actually used as outputs and the software is toggling the pins high and low in a defined manner and each hit of the port latch causes high outputs to be enhanced by the internal short duration strong pullup logic of the quasi-birirectional pins.

There are conveniences with the design of the standard 8051 pin architecture but I largely prefer to use the more modern type derivatives wherein the outputs are push-pull outputs and inputs are true high impedance connections.

Michael Karas