PCI-BUS LIMITATIONS
When transferring large amounts of data through the PCI-bus there are some limitations you should be aware of.

Some facts about the PCI-Bus:

• Runs at 33MHz
• 32-bits wide
• 128MB/sec Maximum Throughput

The above mentioned 128MB/Sec is a cause for a lot of confusion. It is impossible to achieve 128MB/sec sustained throughput on the PCI-bus. There are more than one PCI devices using the bus causing congestion and latency variations. Even if there are no PCI cards in your system, chances are your motherboard has several devices integrated that use the PCI-bus such as HDD controllers, USB controllers, and PCI to ISA bridges. With a minimal PCI configuration (only HDD controller and PCI-ISA bridge) the maximum sustained data transfer you can expect to achieve through the PCI-bus is about 20MB/sec. This is a Bus Mastering DMA transfer from the PCI-Device to system memory only, not to disk. If you are planning to transfer data to disk simultaneously, the most you can expect to sustain is about 1-2MB/sec. It may be possible to increase this a little by using an Ultra/Wide SCSI caching controller and a super fast HDD.

How this applies to our Data Acquisition Cards:
Because of this limitation of the PCI bus some of our cards are not able to perform sustained transfer rates at which they are rated. For example, the PCI-9812 is currently our fastest A/D card, it can sample 4 channels at 20MHz simultaneously at 12-bits resolution. This would mean the PCI-9812 can transfer 160MB/sec of data. As we have seen above, it is not possible to sustain this kind of transfer rate on the PCI-bus. However, you can sample at that speed for a very short period of time, enough time to fill the FIFO on the card. Or in other words, to take a snap-shot of the analog signal at that high rate of speed, then transfer it to memory when the PCI-bus becomes available.

The PCI-7200 is one of our high-speed DIO cards. It has a maximum transfer rate 12MB/sec. This card is 32-bits wide and has a maximum trigger rate of 3MHz. The PCI-7200 supports three input clock modes, internal clock, external clock, and handshaking modes. The first two modes cannot guarantee the input data integrity at high speed data rates because of the limited FIFO depth and PCI-bus latency variations. Handshaking mode is the only way data integrity can be guaranteed. In handshaking mode, you can expect 12 MB/sec data rate in average but not sustained. The sustained data rate with internal clock or external clock mode is 1MB/sec in a machine that a PCI-7200 card is the only device using the bus.

The PCI-7300A is currently our fastest DIO card. It is rated at 40MB/sec and can be configured as 16-bit or 32-bit with a maximum trigger rate of 20MHz. It can achieve a higher sustained transfer rate than the PCI-7200 because of it's deeper FIFO and a new Bus Mastering DMA technology called "Scatter-Gather DMA." This card can sustain 20MB/sec transfer rate to memory and is only limited to the amount of available memory in your system.

In conclusion...
When designing your data acquisition application, keep in mind that you can only sustain 20MB/sec of data through the PCI-bus. This can be accomplished using Bus Mastering DMA, no other PCI cards in the system, and not transferring any data to disk simultaneously. When transferring to disk simultaneously, 1-2MB/sec can be expected but actual results may vary.