After a few updates yesterday citing a technical issue as the reason behind an almost four-hour blackout on its trading floor, the New York Stock Exchange is adding new information about the incident, blaming it on a software upgrade.

In a press release issued today, the exchange operator explained that it had been preparing to roll out a gradual upgrade to its consolidated trading tape ahead of a July 11 industry test. That function tracks buy-and-sell orders, notes MarketWatch. But the upgrade “caused additional communication issues between the gateways and trading units, which began to manifest themselves mid-morning.”

The software upgrade was deployed on one trading unit, and the exchange opened at 9:30 a.m. with the correct software in place. But then at about 11:09 ET yesterday, the NYSE said it was investigating a technical issue and decided to halt trading at 11:32 a.m. as customers continued experiencing “unusual system behavior.”

The NYSE eventually went back online about 50 minutes before the regular close of trading, and the stoppage didn’t affect options trading on either the NYSE or other exchanges like NASDAQ (which continued operating normally).

You can read the statement in full below:

“On Tuesday evening, the NYSE began the rollout of a software release in preparation for the July 11 industry test of the upcoming SIP timestamp requirement. As is standard NYSE practice, the initial release was deployed on one trading unit. As customers began connecting after 7am on Wednesday morning, there were communication issues between customer gateways and the trading unit with the new release. It was determined that the NYSE and NYSE MKT customer gateways were not loaded with the proper configuration compatible with the new release.

Prior to the market open, gateways were updated with the correct version of software and stocks opened at 9:30am. However, the update to the gateways caused additional communication issues between the gateways and trading units, which began to manifest themselves mid-morning. At 11:09am, NYSE issued a Market Status message that a technical issue was being investigated. At 11:32am, because NYSE and NYSE MKT were actively trading but customers were still reporting unusual system behavior, the decision was made to suspend trading on NYSE and NYSE MKT. NYSE ARCA, Arca Options and NYSE AMEX Options were not impacted by this event and continued to trade normally.”

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