I have seen typical bus resistance as few ohms. For example on MAChX02 1200 ZE board the bus that connects FTDI chip and the FPGA has only around 11 Ohm resistance.

Is there any specific reason for keeping bus resistance small?

Thanks in advance


Unless designed otherwise, logic drivers aren't very strong. The process of transmitting a signal can be modelled as charging a small capacitor (a few pf) on the device to be driven, through the resistance of the bus. The higher the resistance, the slower the rise time on the target.

For slow signals you can go up to a few hundred ohms, or insert 100 ohm resistors in a bus if you fear it may be accidentally driven from both ends.

11 ohms seems quite high for a PCB trace.

  • \$\begingroup\$ Thanks. But Can you elaborate " logic drivers aren't very strong" what do you mean by this. Rest is clear. thank you very much \$\endgroup\$ – gpuguy May 10 '13 at 14:58
  • \$\begingroup\$ The output pins of digital logic devices are connected to small MOSFETs. They don't source or sink large amounts of current. \$\endgroup\$ – pjc50 May 10 '13 at 15:16

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