How to select a good optocoupler for uart to 20 mA current loop converter?

Question

I'm trying to make a circuit that converts UART signals to 20 mA current loop signals. I tried this circuit to switch the signals.

I am not much experienced in optocouplers I had just two types to test, TLP251 and 4N25. The below pictures show voltage of Node1 on oscilloscope. The input signal on TXD is a 20 KHz sqaure wave.

What is a good choice for optocoupler which is capable of transferring signals up to the 1M Baudrate of the uart?

This is what I got with 6N137.

The final Circuit in my mind would be something like below:

I need the logic to be the same not inverted which mean 5V on TXD makes 20 mA on current loop and 0V makes 0 mA.

Answer 1

A simple current source with mirror. Google to find the calculation for correct resistance. The Q1 needs to conduct the N-th current of parallel transistors, where N is the number of mirror's parallel transistors.

Why current source? Because the current loop is to be used on long distance cables and low speed. The current source will compensate for the long cable resistance.

^{simulate this circuit – Schematic created using CircuitLab}

Try this:

^{simulate this circuit}

The S5 PLC already has two current sources for that purpose.