ISO 8583

First published in 1987 and revised in 1993 and 2003. Every message opens with a four-digit message type indicator (MTI): 0100 is an authorization request, 0110 its response, 0400 a reversal and 0800 a network management message that mostly exists to prove the line is still alive. Next comes a bitmap, each bit flagging which of up to 128 data elements follow. Field 2 is the card number, field 4 the amount, field 39 the response code, where 00 means approved.

It was designed for expensive links and counts every byte. It is also more grammar than language: Visa, Mastercard and every processor speak their own dialect, so two systems that both "do ISO 8583" rarely understand each other without a translator in the middle. Account-to-account payments have moved to ISO 20022. The card world shows no sign of following.