How can I make a UART receiver using logic devices 74164,counters,logic gates,..

Hello, I am trying to make a Serial-in Parallel-out Register to be controlled by my PC with logic gates and a 74164, 74193, 555 timer. I am programming it through visual basic and I have no problem making the serial communication, I even made a chat (in a prior application) with it and the data is sent correctly but I'm having some issues with receiving the data and keeping it with the logic. The idea is being able to control the data I output to then control a stepper motor.



This is the circuit I have developed so far, the 555 timer has 10,200 hz frequency the serial communication is 9600 bauds, the counter is going to stop at 9 and the clock is going to be activated when a 0 logic so it is starting when the start bit arrives. I have a NAND flip flop to store when to start and it is going to be deactivated when the counter reaches 9, the problem is it is really bugged and I am not sure why it is, when I send any data the 74164 displays 127 or 63 in decimal (0111 1111 and 0011 1111 respectively) and I would like to see if you could help me solving it.

Where to start?
You draw the connector as if it is working with RS232. The signal for RS232 is bipolar. With modern circuitry the voltage levels have been reduced and about -5V for a mark and +5V for a space. It is possible that the standard even allows for =/-3.3V. I see no provision in your circuit for this.

If the baud rate is 9600, why would you consider a different frequency (on the 555)? Your problems are even deeper than that, but I don’t follow your logic. UARTs normally require that the baud rate is within +/-5%. Even if you could tune the 555 to the right frequency, it’s unlikely that it would be a good long term solution. UARTs typically work at multiples of the baud rate, 8X or 16X and they sample through a full bit time and average it to decide on whether it is a more or a space.

I would suggest that you look into some early UART data sheets to get an understanding how they work. The IM6402 might be a good place to start.

And then, in an ironic twist, you might want to look at software UARTs (google it) to see how they work and then implement it in hardware.

But make sure that you translate the input voltage to your logic levels and then invert it so that a mark is “high” voltage.

I should also have mentioned that if you are only going to sample the input once, it should be in the middle of the bit. Multiple samples would we distributed over the full time of the bit, but avoiding bit transitions.