Mini-Split Communication

[Husky250]

Hello Everyone,

On most Inverter systems I have worked on, I discovered both Indoor and Outdoor units are high voltage.

My question is the 3rd wire used for communication. Can anyone explain how this communication occurs, and can it be used to check if Indoor and Outdoor PCB'S are functioning? I do know power wiring crossed between Indoor and Outdoor units will cause communication problems. I am assuming thats because voltage is being used (or converted) to DC for communication. Is this theory correct?

Thamks to all!

[allanIIIrd]

the communication between outdoor and indoor unit is done by modulating the frequency of the communication line.(in this case the 3rd line)because voltage drops in long span of wire and the longer the line the more resistance it has to ampere which makes modulating the frequency much more plausible.

[cadillackid]

most mini splits use Pulse distance encoding.. which is the same technique that IR remotes use...

the line between the indoor and outdoor units is typically 18 to 60 volts.. (depending on the manufacturer)..

the 5 volt TTL level signal from the micros is converted to a Higher voltage signal and pushed through Opto-isolaters...(higher voltage is less likely to succomb to EMI and small spikes.. the opto's protect the Micro processors in the event of Larger Spikes or lighting or mis-wiring by technician.

the indoor board is typically the Master controller in the system.

the theory behind is as follows:>.

1] it is simple highs and Lows..
2] all data bits are sent as Highs (or lows if it is an inverted communication.

so for instance the a Logic High on the line for .768 milliseconds might be a Binary 0, and a High on the Line for 2.1 milliseconds might be a Logic 1. (this being for the indoor to outdoor unit.
a low of .768 mlliseconds indicates a space between bits..

typically there will be a Long start bit that is a high of say 30 milliseconds to signal the other end that the line is being seized...

so the indoor board constantly sends out its stream of data.. typically it is 4 to 8 Bytes of 8 bit binary words.. it sends 1 byte, then inverts all the 1's and zeros and sends it again.. this is for error checking..

if the receiving unit sees the original but the second does not match on the inverted it throws out the data...

at the end of a data stream there will typically be a stop bit of length half of the time of the start bit...

next the outdoor unit will pull the line low for a certain time (100 milliseconds is common).. this signals the indoor unit that the outdoor unit is going to respond..

the outdoor unit sends its data stream through in similar fashion as the indoor unit.....

and then the cycle starts all over...

if the indoor unit does never get a response from the outdoor unit in say 2 seconds.. it will re-send its data string...

typically after 5 sends and no receipt of good data the indoor unit will issue an error code for failure of communications and will shut itself off...

the outdoor unit if it does not receive any comms from the indoor unit in an alotted time period will automatically go dormant and shut down the compressor and fan until it receives good comms again...

many units are different... what I explained is common on LG and CHigo(grunaire, turboair,americaire,amcorair), Mitsi uses a little different technique as does daikin...

some of the older units actually had 2 lines and had standard RS-485 serial protocols...

communications between indoor and outdoor units can be intercepted and monitored / altered.... Intercepting it with a micro-controller for diagnostic monitoring is a great way to see inside of your unit and what it is doing....

since I do a lot of controls work with Chigo based units, i have a Modified indoor board that talks to my laptop so I can diagnose those near impossible to fix issues on the units...

-Christopher