How to control a RCX from a Mindstorms NXT I have both an RCX and an NXT.
I would like to control the RCX brick from the NXT (and possibly communicate back to to the NXT from the RCX). I would like to avoid purchasing the Mindsensors RCX to NXT Communication Adapter which is no longer available in their store, and instead control the RCX via inputs to one of it's sensor ports via outputs from an NXT sensor port. 

The physical connection can be achieved with the NTX to RCX converter cable but I am unsure how to control the output from the NXT sensor port to emulate any of the sensors that the RCX can recognise. 

I found this post on another forum asking a very similar question, but the people who responded don't seem to have understood OP's intent, it is however worth a look for the illustrations.

This then is really a question about programming the NXT to gain fine control over the output from a sensor port to emulate a RCX sensor and if it is even possible.

I am open to solutions in any programming language/environment.
I think you're overlooking a very simple yet elegant solution: connect a NXT motor to the motor port (no problem there, that's what it's for), connect a RCX rotation sensor to the RCX (again, the intended purpose), and by now you should pretty much know where I'm going with this: connect the rotation sensor mechanically to the NXT motor.

Considering the NXT motor has a good precision, you can easily you this to send messages from the NXT to the RCX; but of course you'll need to invent the protocol yourself; that is, decide what these messages mean. And you won't have feedback from the RCX to the NXT (you didn't ask for it though)



I know you don't want to purchase anything, but for reference, there is now the IR Link Sensor made by HiTechnic and sold directly by LEGO, which is presumably easier to use (and you can get feedback through it, too).

I don't think you'll be able to use a converter cable directly; you'll need to use some intermediary electronic, possibly using the I2C protocol. This prototype board solution (also from HiTechnic) should do the trick if you use one of the outputs to drive the RCX input (you can probably use an analogue one).

And if it's more ports you're after, you'll want to consider their multiplexers too.