ELECTRICAL PROPERTIES
With the exception of the contact unit, a terminal primarily consists of electronic components. These are used to provide the interfaces to the user and the background system, and to electrically drive the contacts. The terminal’s electromechanical parts and the smart card itself must be supplied with electrical signals. The only information that is directly provided by the contact unit is whether a card has been inserted. The only signal that is sent directly to the contact unit is the signal to actuate the automatic card ejector, if such a device is present. The card interface consists of the five contacts for the ground, supply voltage, clock, reset and data signals. Once the electrical connections have been made, it is very important with regard to the service life of the card for the activation sequence specified by ISO/IEC 7816-3 to be followed exactly. Otherwise, the chip may be electrically overstressed, which will increase the failure rate. It is also important to observe the proper deactivation sequence, since otherwise the same problems may occur. In this regard, there is an important consideration with simple terminals that allowthe user to remove the card manually. Whenever the contact unit detects that the card is being withdrawn, the terminal’s electronic circuitry must immediately execute a deactivation sequence. This is the only way to prevent the contacts from sliding across the contact field of the card while they are possibly still energized, which would produce results that have little in common with a standard deactivation sequence. However, the consequences of such an unallowed card withdrawal can be even more serious, since shorts may occur between the leads if the contacts are worn or slightly bent. The mild sparking due to the discharge of capacitors in such a situation will damage both the contact elements and the contact surfaces of the card. With regard to the electric circuitry, almost all terminal manufacturers have realized by now that short-circuit protection is indispensable. If this point is neglected, a single smart card with shorted contact surfaces can cause the electrical demise of very many terminals. Incidentally, shorted cards crop up regularly, partly due to vandalism and partly due to technical defects. Short-circuit protection should extend to the point that every contact can be connected to any other contact or group of contacts without any repercussions. Ideally, the circuitry that drives the smart card should be fully electrically isolated from the remaining circuitry of the terminal. This is standard practice in public card phones in Germany, since it also largely protects the equipment against externally applied voltages as well as shorts.

The voltage needed for writing and erasing EEPROM pages is generated by the microcontroller via a charge pump on the chip. This can draw currents of up to 100 mA for intervals of a few nanoseconds. The same effect, in a reduced form, can be produced by transistor switching processes in the CMOS integrated circuits. Even very fast regulator circuits in the power supply cannot handle these short spikes, with the consequence that the supply voltage for the card collapses due to the heavy current load and the EEPROMwrite or erase cycle fails. In extreme cases, the voltage dropout can be so severe that the processor lands outside of its stable operating area and a system crash occurs. The remedy is to connect a capacitor as close as possible to the contacts for the smart card. A ceramic capacitor of about 100 nF is suitable, as it can release its charge very quickly. The leads to the smart card must be as short as possible, so that lead resistance and inductance do not significantly affect the ability of the circuit to meet the increased current demand within the necessary interval. A brief increase in current demand can be met by drawing charge from the capacitor until the voltage regulator can respond to the change. This is a simple and economical way to avoid power supply problems. Particularly for electronic payment systems, it is nowadays standard to equip the terminal with a real-time clock. This is required for reasons of traceability and user protection. According to the EMV specification for credit card terminals, the clock may not be off by more than 1 minute per month. This is not technically difficult, since suitably accurate clock components are available as single-chip solutions. In addition, the clock can be adjusted every time the terminal makes an online connection to the higher-level system. Radio time signal receivers in terminals have so far not achieved any practical importance, since signal reception is too strongly affected by the screening effects of the site where the terminal is installed. Standard time code signals usually cannot be received inside a reinforced concrete structure, for example.

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