2w ago

Questions on how to connect a PC to a washing machine (USB→TTL serial) ← UPDATE 2: the serial port has power - but is it functional?

I need to connect a PC to my washing machine. The washing machine has a bank of pins labelled like this:

The microcontroller is an ATmega32L, which has specs for the serial connection as follows:

My USB to TTL serial adapter is apparently based on a ch340 chip. It looks almost exactly like the pic I attached, except mine does not have a crystal on it because I think the chip has an embedded clock. The important thing is the pins match my adapter.

My knee-jerk thought was to connect it as follows:

adapter → washing machine PCB

gnd → 0v
rxd → tx
txd → rx
3v3 → (nothing)
5v → 5v ← bad idea?

(with s1 jumped to 5v on the adapter)

Someone told me I should not connect 5v to 5v. I was assuming one connection needed 5v and the other supplied it, but I was told they are both supplying 5v, but not perfectly 5v, so the difference will strain something and cause damage.

So how should I hook this up?

update (I’m stuffed?)

I heard washing machine manufacturers often sabotage the serial ports before shipping as an anti-repair tactic. I thought my old machine might pre-date that practice, but I might be wrong. I metered TX voltage against 0v using a crappy cheap DMM. Results:

0.01, 0.00, 0.01, 0.00, 0.01, 0.00, 0.01, 0.00, … etc, every second or so.

Looked encouraging, as if there is activity. Then I metered 0v against 5v:

~~0.01, 0.00, 0.01, 0.00, 0.01, 0.00, 0.01, 0.00, … etc, every second or so.

Yikes. I was expecting that to read a steady 5v. Due to bad wiring in the house, I think that is just noise on the ground wire. And apparently the serial port is dead.

I had the two 5v lines connected to each other for a while, so it’s possible I damaged it, if not the manufacturer.~~

Update 2: I have 5v, so the port may work

It turns out I had a bad alligator clip, so of course I got a flat reading. The 5v pin on the serial port gives 5v. Thus I might have a live port. Now how do I use it?

6 comments

If they both are power supplied, and have ground connected then what would you need the 5V connection for? Some devices that externally supply power can come with a protection diode to prevent looping power back to prevent the scenario you described but looks like that's not needed. You would have to know the protocol used such as baudrate, bitlength, stopbits and parity. If you don't have that info try common baudrates (115200), 8 bits, 1 stop, no parity.
- Note: another really common combination is 9600 baud, 8 data, 1 stop. Antiquated, but a washing machine probably doesn't need anything fast.
- I had the two 5v pins connected (from 5v on the adapter to the 5v pin on the washing machine). I had it that way for maybe 10 or 15 min until I was told not to. The USB→TTL adapter was a little warm when I disconnected it. Now I wonder if I damaged the washing machine port because when I meter the 0v against the 5v, there is almost nothing there. Did I damage it, or did the manufacturer disable the serial port before it got to me?
  
  Hard to say, but if the 5V natively to the machine is shot, I'd expect the unit to not work as its systems would depend on it. I assume you measured 5V in reference to the machines GND. Double check your meter against a known good 5V supply (your adapter).
  
  I assume you measured 5V in reference to the machines GND.
  Indeed. It was in fact the 0v pin on the washing machine that is next to the RX pin.
  When I meter gnd against 5v on the adapter, I get 5.15v. So both the adapter and the DMM are fine.
  I heard that connecting two DC supplies together would have no problem if they both output exactly the same voltage. Of course we would never have an exact match, but the only strain on either side of the connection would be from the difference between the 5v from the adapter and 5v from the washing machine.
  ~~So I’m tempted to conclude no damage was done and the serial port was sabotaged at the factory.~~
  (edit) The w/m also gives 5v. Turns out my alligator clip was bad. So the port may be fine.