Three more companies have announced card readers that work with Apple Inc.'s iPhone.

Hypercom Corp. announced Tuesday that it would sell two versions of a card reader that plugs in to an iPhone and uses its wireless connection to handle card payments. The SmartPayments Mobile devices are aimed at merchants who want to accept card payments but cannot be tied to an office.

One version of SmartPayments Mobile includes a bar code scanner to further replicate the experience of paying at the point of sale. Hypercom expects to begin selling these readers in June.

Also Tuesday, MagTek Inc. announced its own iPhone card reader. MagTek began selling this week its iDynamo, which uses the company's MagneSafe security system for generating dynamic data from magnetic-stripe cards.

And last week, GorCorp Inc.'s payment gateway USA ePay introduced its own iPhone card reader, which it calls PaySaber. Like HyperCom's reader, USA ePay's reader lets merchants scan bar codes. USA ePay also sells versions for phones that run BlackBerry, Windows Mobile and Android software.

Though many payment terminal makers already offer mobile card readers, observers say that accepting payments with smart cards could be more cost-effective because merchants need not pay for a wireless data connection beyond what they already pay for using the phone itself.

The market for iPhone card readers is getting crowded.

The start-up Square Inc. kicked off the iPhone card reader trend with its December announcement of a device that potentially could plug in to the audio port of any smart phone. Despite this early move, Square was beaten to market by VeriFone Holdings Inc., which sells an iPhone-specific reader called PayWare Mobile.

Square now offers payment software only for Apple's new iPad tablet; it only connects through Wi-Fi; a version that communicates through AT&T Inc.'s wireless network is planned for this month. Square has not offered an iPhone version of its product.

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