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Each hash board has default frequency and hash rate. If the default frequency and hash rate of hash boards are the same. You can mix them.
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The reality is that you can mix together all S9 hashboards and even with R4 hashboards.
But do not be put on a higher frequency which is on the weakest hashboard. 550M
In this miner working only 2 hashblades. The third hasboard is not connected.
One from this working boards is 650M (VER3) with all heat sinks the second hashboard is with missed radiators 550M (VER6)
controller software is for 650M miner.
Which hashboard is 550M and which is 650M?