PCI Spec stipulates that up to 32 PCI devices can be connected to each PCI bus, but in fact it is not up to 32. The 33 MHz 32-bit PCI bus can only connect 10 to 12 loads.
Note: If a slot connection is used, one connection counts as two PCI devices, and the slot and PCI card count as one PCI device, respectively. In other words, a 33MHz PCI bus can only connect up to 4 to 5 slots or PCI cards.
This is because when the PCI bus is designed, in order to reduce power consumption, a technique called reflected-wave signaling is used, as shown in the following figure:
From the figure, we can see that in order to reduce the power consumption, the transmit end of the PCI device uses a weak transmit buffers, which can only drive the signal level to half of the actual demand. Then rely on the signal reflected back superimposed on the original signal, so that the signal level to the actual needs. Of course, all these processes are required to be completed within one clock cycle. This mechanism also limits the increase of the PCI bus frequency and also limits the maximum number of connected devices on a single PCI bus. If you need to connect more PCI devices, you need to use the PCI-to-PCI bridge. Each bridge has its own internal isolation, which ensures that each bridge can connect an additional 10 to 12 loads. However, PCI Spec specifies that up to 256 sub-buses can be used in a PCI bus system.
In addition, the input buffer of the PCI bus has not been added to the input register, which imposes higher requirements on the signal setup time.
The architecture of a 33 MHz PCI bus system with a PCI-to-PCI bridge is shown below:
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