ISO 7816 Part 2: Dimensions and Location of the Contacts
ISO/IEC 7816-2 answers one narrowly defined but essential question: where on the card are the electrical contacts, and how big are they? It is what makes a card issued by one organisation work in a reader built by another.
The interoperability problem it solves
A reader makes contact through spring-loaded pins in fixed positions. If a card’s contact pads were positioned by each manufacturer’s preference, a pin could land on the boundary between two pads, or off the plate entirely. Part 2 fixes the geometry — the contact area’s position relative to the card edges, and the extent of each individual contact — so that any compliant pin set meets any compliant pad set with margin to spare.
The contact assignment
The contact plate carries eight positions, conventionally labelled C1 to C8. Their functional assignment is the piece most developers actually need:
- C1 – VCC. Supply voltage.
- C2 – RST. Reset signal from the reader.
- C3 – CLK. Clock supplied by the reader; the card has no oscillator of its own.
- C4 – reserved / auxiliary. Used by the USB electrical interface defined in Part 12 on cards that support it.
- C5 – GND. Ground reference.
- C6 – VPP. Programming voltage on legacy cards; not used by modern cards, which generate what they need internally.
- C7 – I/O. The single bidirectional, half-duplex data line.
- C8 – reserved / auxiliary. Also used by Part 12 for USB.
Two things about that list are worth dwelling on. First, there is only one data line: C7 carries traffic in both directions, which is why every protocol in this family is half duplex and why the card and reader must never transmit simultaneously. Second, the card is entirely passive: it has no power source and no clock, so it does nothing at all between the reader energising VCC and releasing RST.
Practical implications
- Contact plate size varies, position does not. Modules come in different footprints, but the specified contact locations must still be met. Cosmetic module choices are constrained by this.
- C4 and C8 are not free real estate. Using them for a proprietary purpose forecloses USB support and can misbehave in readers that drive them.
- Artwork must respect the contact area. Printing, embossing and signature panels have to keep clear of it, which is a card-design constraint discovered late more often than it should be.
- Debugging tip. Intermittent failures in one reader model and not another are frequently a contact geometry or pin-wear issue rather than a protocol issue. Swap the card and the reader independently before touching firmware.
Relationship to the rest of the series
This page is part of Ambimat’s ISO 7816 smart card standard developer reference.
Part 2 defines where the pins land. What the card body around them must withstand is ISO/IEC 7816-1; what the signals on those pins mean is ISO/IEC 7816-3; and the USB electrical interface that reuses C4 and C8 is ISO/IEC 7816-12.
Authoritative source
For the normative specification, refer to ISO/IEC 7816-2:2007, Identification cards — Integrated circuit cards — Part 2: Cards with contacts — Dimensions and location of the contacts, published by ISO. This page is Ambimat’s own explanatory summary and does not reproduce or replace the standard.