​Electronic engineer explains the wiring rules of PCI card printed circuit board

Circuit board assembly and circuit board processing manufacturer explain the wiring rules of PCI card printed circuit board

The cabling of PCI card is more particular, which is determined by the characteristics of PCI signal.

In conventional high-frequency digital circuit design, we always strive to avoid signal reflection, overshoot, ringing, and non monotonicity caused by impedance mismatch. However, PCI signals precisely use the reflection principle of signals to transmit physical signals. In order to make rational use of signal reflection and try to avoid large overshoot, ringing, and non monotonicity and other side effects, PCI-SIG has made some provisions on PCB physical implementation in PCI specifications.

PCI-SIG recommends that PCI cards use four layers of PCB boards. The signal distribution of PCI connectors specified by PCI-SIG is also optimized for the convenience of four layers of board wiring. PCI-SIG has also made a recommended schematic diagram for the pin distribution of the PCI controller. In fact, the manufacturers of PCI controllers such as AMCC, PLX, and OXFORD have also implemented this recommendation. Under this recommended pin distribution, it is actually convenient to use a two-layer PCB board for wiring. However, if the hardware of the PCI card system is complex and multiple power division layers are required, multi-layer PCBs are better.

Any PCI signal on the PCI card can only be connected to one load (including a pull-up resistor). Except for CLK, RST, INTA #~INTD #, JTAG, all pins from the contact point between the golden finger and the clamp base to the load end shall not be greater than 1.5inch; The CLK signal length is 2.5 ＋－ 0.1inch, which is a little long. Therefore, in many cases, the cable needs to be twisted to meet the length requirements. This is why the CLK serpentine cable is often seen on the PCI card; There are no special provisions for the other pin. When multi layer PCB is used, the signal routing should not cross different power supply layers (at least, the layer with the split power supply layer should be located on the other side of the PCB), which is why it is often seen that all signals coming from the gold finger on side A of the PCI card are often punched to the side B (component plane).

The characteristic impedance of each PCI signal is 60~100 ohms, the load capacitance shall not exceed 10pf, and the IO Pad of IC shall be able to withstand - 3.5V undershoot and+7.1V signal overshoot. For manufacturers of PCI controllers such as AMCC, PLX, OXFORD, etc., their controller ICs meet these regulations, and users do not need to consider them. However, if CPLD/FPGA is used to implement PCI controllers, you must consider whether the model used meets these regulations. Generally, manufacturers of CPLD/FPGA such as Altera, Xilinx, etc. will clearly state in their data manuals whether this model CPLD/FPGA is compatible with PCI signal specifications.

Well, the wiring of an ordinary 32-bit 33MHz PCI card is relatively simple, mainly meeting the length requirements. In fact, if the cabling requirements are not strictly followed, there will be no problem. However, depending on the motherboard chipset, once the signal compatibility problem arises, it will be the most painful experience in circuit design to hardware debug the PCI card. The circuit board assembly, circuit board design, and circuit board processing manufacturer explain. The electronic engineer explains the wiring rules of the PCI card printed circuit board.