backplate. This allows cards to be fitted only into slots with a voltage they support. "Universal cards" accepting either voltage have both key notches. Connector pinout The PCI connector is defined as having 62 contacts on each side of the edge connector, but two or four of them are replaced by key notches, so a card has 60 or 58 contacts on each side. Side A refers to the 'solder side' and side B refers to the 'component side': if the card is held with the connector pointing down, a view of side A will have the backplate on the right, whereas a view of side B will have the backplate on the left. The pinout of B and A sides are as follows, looking down into the motherboard connector (pins A1 and B1 are closest to backplate). 64-bit PCI extends this by an additional 32 contacts on each side which provide AD[63:32], C/BE[7:4]#, the PAR64 parity signal, and a number of power and ground pins. Most lines are connected to each slot in parallel. The exceptions are: Each slot has its own REQ# output to, and GNT# input from the motherboard arbiter. Each slot has its own IDSEL line, usually connected to a specific AD line. TDO is daisy-chained to the following slot's TDI. Cards without JTAG support must connect TDI to TDO so as not to break the chain. PRSNT1# and PRSNT2# for each slot have their own pull-up resistors on the motherboard. The motherboard may (but does not have to) sense these pins to determine the presence of PCI cards and their power requirements. REQ64# and ACK64# are individually pulled up on 32-bit only slots. The interrupt pins INTA# through INTD# are connected to all slots in different orders. (INTA# on one slot is INTB# on the next and INTC# on the one after that.) Notes: IOPWR is +3.3 V or +5 V, depending on the backplane. The slots also have a ridge in one of two places which prevents insertion of cards that do not have the corresponding key notch, indicating support for that voltage standard. Universal cards have both key notches and use IOPWR to determine their I/O signal levels. The PCI SIG strongly encourages 3.3 V PCI signaling, requiring support for it since standard revision 2.3, but most PC motherboards use the 5 V variant. Thus, while many currently available PCI cards support both,