it is amazing how i managed to be 17 for 17 on my s17pros. still all work just fine.
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It's also amazing how many problems are reported by others about the BM 17 range.
Yesterday I had a deal for 821s @85 CAD.
I'd say take it. The 821 is only about 1-2 THs slower than the 841 though again, remember you cannot mix different models on 1 controller.
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Also important to note that running too many miners on one controller can have a noticeable increase in stale shares.
The miner USB has a limit to now many I/Os it can perform per second, which is separate to the total Bytes/second.