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12 September 2006 V104 Manual
Tri-M Engineering Tel: 800.665.5600, 604.945.9565
1407 Kebet Way, Unit 100 Fax: 604.945.9566
Port Coquitlam, BC V3C 6L3 E-mail: info@tri-m.com
Canada Web site: www.tri-m.com
18
APPENDIX 1
Advantages of Using AC Termination
One of the requirements of embedded electronics is for low power consumption. One method
of reducing power is to reduce the drive current available to power the expansion bus. With
over 80 signal lines, any reduction in current load would have a large impact on overall
requirements. The PC/104 Consortium Guidelines for the expansion bus specify drive current
can be as low as 4mA. Compared with the 24mA for the standard desktop computer, this is an
84 percent reduction in the drive current available.
The disadvantage to reducing drive current is the increasing possibility for noise to infilitrate
the bus. The symptoms of noise induced problems are often “flaky” or unreliable operation.
Systems suffering from noisy busses are often difficult to diagnose and solve. Programmers
blame the hardware engineers and the hardware engineers blame the software programmers.
With reduced drive currents, more attention must be paid to reducing the noise levels on the
PC/104 bus. One frequently used method is bus terminators. Testing has proven the best
way to terminate the PC/104 bus is to use AC terminators instead of resistive terminators.
This is the recommended termination method for Ampro CPU products. The IEEE P996 PC
Bus Standard recommends the AC bus terminating technique.
The use of AC terminators has several advantages over DC terminators:
- Reduced power consumption: DC terminators are typically in the 330 ohm to 1K ohm range
and draw heavy currents. This is significant when terminating the over 80 signals on the
PC/104 bus. AC terminators draw current only during the few nanoseconds when the bus
signal is changing state, resulting in negligible current drain.
- Improved bus reliability: DC terminators invariably increase the voltage level of the logic
zero state. This decreases noise immunity, making it more likely a zero will be seen as a one.
AC terminators do not cause this shift, resulting in a more reliable bus.
- Reduced crosstalk: AC terminators roll off the signal transitions on the bus. The result is a
quieter bus which has fewer high frequency effects such as crosstalk to other bus lines.
- Reduced EMI. Busses with AC termination tend to generate less EMI than resistively
terminated buses due to the reduction in high frequency components of signal transitions.