Debit / credit pinpad terminals disconnect from network after 15 mins. Reconnect after one error

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http://serverfault.com – We have an HP procurve network and about 20 of the standard debit/credit pinpad terminals everyone is used to seeing at just about every store these days. They connect directly to the LAN and only communicate out to a payment site on SSL/443. No software or server in the middle. The problem is that the devices usually give a TCP connection failure on the first attempted use. They'll then work fine for an hour straight. But, if allowed to sit idle for 10 - 15 mins (approx), they'll throw the initial error once. Initially, they were all from a single company and we figured it had something to (HowTos)