Serial communication protocol
These are just a couple of pointers to help you.

Forget your 555 idea. Use something that can generate any byte value from 0 to 255. For your learning to be useful you will need to develop some micro-controller skills so you might as well start now.



Figure 1. Basic principle of serial data transfer. One byte of data is transferred out one bit at a time, LSB (least significant bit) first.

Be aware of the need for start and stop bits and how they are used to synchronise the receiving device clock for correct decoding.



Figure 2. Receiver sampling points. Source: Sangoma.

The start bit is used to trigger the read cycle in the receiver. The receiver synchronises itself on the start bit and then waits 1.5 cycles to start sampling bits. Thereafter the bits are sampled at the baud rate. This initial delay means that even with a 5% clock error the receiver should still be within the bit timing for the last bit.

Since the start bit - shown low in Figure 2. - is identified by a falling edge then it must be preceded by a high and this is what the stop bit ensures. The alternative would be two start bits and no stop bits but it wouldn't change the total message length.

The linked article has some other points worth noting.

If you can make an RS232 transmitter you could monitor the data using a program such as PuTTY on a PC with serial interface or RS232-USB adaptor.