The New York Stock Exchange says a planned software update caused Wednesday's system shutdown that halted trading for more than 3.5 hours.

In a statement, the NYSE says its problems began Tuesday night with the rollout of a software release. The exchange says that as customers began connecting after 7 a.m. ET on Wednesday, "there were communication issues between customer gateways and the trading unit with the new release."

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The NYSE says it turns out the gateways were "not loaded with the proper configuration compatible with the new release."

The NYSE updated the gateways with the correct version of the software. But the gremlins weren't done yet.

The update "caused additional communication issues between the gateways and trading units, which began to manifest themselves mid-morning," the exchange says.

That's when the exchange halted trading. The NYSE said it then "began the process of canceling all open orders, working with customers to reconcile orders and trades, restarting all customer gateways and failing [sic] over to back-up trading units located in our Mahwah, NJ datacenter so trading could be resumed in a normal state. "

The NYSE notes trading resumed shortly after 3 p.m. Wednesday.

And the NYSE is functioning as normal today.

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