PCI-X Digital I/O

The PCI bus in the form that is used today was first introduced in 1995. The last years it has been the most common platform for PC based instrumentation boards. Nowadays PCI based systems are more and more superseded by PCI Express based systems.

Its world-wide range of installations, especially in the consumer market, still makes it a platform with good value. Based on the PCI bus the PCI-X bus was specified for applications needing a higher data throughput. On the PCI-X bus there are bus frequencies up to 133 MHz and data bus widths up to 64 bit available. The M2i and M3i cards use the PCI-X bus with 66 MHz to gain a high data throughput. All PCI and PCI-X cards from Spectrum are compatible to PCI as well as to PCI-X with 33 MHz up to 133 MHz bus frequency.

Different Slot Types

The PCI bus as well as the PCI-X bus uses a voltage coding inside the slot to determine between 5V and and 3.3V I/O signals. All Spectrum cards of the M2i and M3i series are universal cards and can operate with both I/O voltage levels. There are some very early Spectrum cards of the former MI series that only allow 5V I/O signals.

The PCI-X bus slot are extended by additional lines to allow 64 bit wide data transfer. In addition the PCI-X slots are available in different speed grades with 66 MHz, 100 MHz and 133 MHz. The available speed depends on the structure of the motherboard and can be found inside the motherboard manual.

The Spectrum cards use 32 bit wide transfer and match into any PCI-X slot. M2i and M3i cards can operate at 33 MHz and 66 MHz while former MI series can only operate at 33 MHz.

PCI/PCI-X Bus Restraints

As the PCI bus is a parallel bus, meaning that all slots of one bus segment are connected together there are some restraints:

• All cards in one bus segment can only operate at a speed that the slowest card in that bus segment defines. If a Spectrum M2i or M3i card is plugged into a bus segment where an older card with 33 MHz bus speed is working, the Spectrum card will also only work with 33 MHz.
• The sum of all data transfer is limited to the speed of the bus segment. 4 cards that try to stream data in one PCI bus segment at the same time will each only stream at 1/4 of the maximum bus speed.