UART for board to board communication in presence of 220 VAC wires

Question

I am working on a product that is essentially a combination of below shown two products:

There will be a keypad and a separate box containing relays and power supply. A cable will run between the two for power (5 VDC) and signals (3.3 V logic). You can press a button on the keypad and this will turn a relay on/off.

Block diagram is as shown below:

I want to use a separate MCU for the relay module in order to minimize the number of connections between keypad and relay box. Keypad can communicate over UART to the relay module.

Since relay module will also connect to 220 VAC wires, there is a very high probability of communication cable running parallel to ac wires.

My questions are as follows:

1. Will UART work well in such conditions? Its ok if a couple of messages get corrupted. MCU will realize this and packet can be resent.

2. Are there better alternatives to this approach that come for low cost and don't take a lot of PCB area?

3. Do I need to worry about induced noise damaging anything? Should I put some TVS diode on all signals?

Some clarifications:

1. I can't make a single assembly of both PCBs due to some mechanical constraints. Also, I would want to use same keypad for 2-3 different designs of relay module.

2. I can't use i2c port expanders on back end because my other variants might need to performs some logic on the relay board and MCU is coming for a low cost.

Answer 1

I have done something similar, and even though 30cm doesn't sound long, running in parallel to AC wires is likely to cause noise.

To address your 2nd and 3rd questions.

You can take an approach of incremental "hardening" of the design and see which one works.

1. The easiest thing to do is use a shielded cable to go between the two boards. Using shielding combined with lower baud rates may be enough to make it, especially if your software is smart enough to use a checksum or something and simply retry if messages get clobbered. In terms of protection, regular AC induced across the wire could hurt a 'sensitive' mCPU as some of these pins don't do well with voltages going over 3.3V. Others have built-in diodes which should offer some protection. I personally would not be comfortable with this option in terms of damage, especially during storms.
2. Maintain TTL-level UART but use a driver chip, especially one that can drive 5V from the 3.3V and vice versa. You still have to use shielded wire but the driver chips have much better immunity to noise as they are made for that.
3. Use RS-232; the advantage is that you maintain the same number of wires but it should be almost immune to noise In terms of protection, it will depend on the specific transceiver you use but it is pretty good. The disadvantage is that you now need transceivers on both sides. Shielding is recommended, but optional.
4. Use RS-422; if you find the right transceiver, you will get the most protection as some of these chips are designed to handle hundreds of volts in spikes. Differential signaling means that noise immunity is very high. You will still need transceivers on both sides and extra wires.

I highly recommend you go with option 3 or 4. As an advantage, if (or rather when) the 30cm ever need to become 300 cm (or more) you are still covered.