The AFOS PC
A previous investigation into DSP PC104s resetting upon power up (13/4/99) had noted the susceptibility of the
PC104 to supply line impedance.
This investigation checked the vulnerability of the PC104 card when used in the AFOS and fed from an ISA
backplane. The integrity of the bus signals was also examined.
The AFOS PC104 card is mounted on an ISA backplane adapter, and the CCD card fitted in the ISA backplane –
although no connection was made to the CCD for these investigations.
Current consumption was 1.95A (of which 1.25A is used by the PC104 and 0.7A by the CCD card). Voltage
measured on the PC104 was 4.88V compared with 4.92V on the ISA bus and 5.06V at the supply. This indicates a
supply line impedance of 0.1. The Oriel shortform catalog indicates that the current consumption of the CCD card
and CCD is 1.25A with an extra 1A for the peltier cooler – resulting in a maximum current consumption of 3A. At
this maximum current consumption, with the present power supply arrangements, the voltage on the PC104 card
will be 4.65V – the switching point of the MAX 691 power supply monitoring chip. Power line impedance should
be reduced to as low a practical value as
The ISA backplane has several connectors that may be used for the power connection – using these in parallel will
lower the supply line impedance. Similarly, the PC104 processor card has its own plug for 5V power, which could
be used in parallel with the supply via the ISA to keep supply impedance within acceptable bounds
The 4 of 1000uF capacitors would be better replaced with one l0000uF 10V.
There has been some concern that the PC bus signals are being de-graded in going from PC104 to ISA. Several
signals were examined on a scope and it was noted that they were generally of a poor shape displaying significant
undershoot (-0.5V) and noise.
Many manufacturers of ISA backplanes provide provision for terminating resistors. The IEEE P996 specification is
common for the PC104 and PC ISA bus (unfortunately the spec is not available free), this provides for optional
termination of 40-60 in series with 30-70pF connected between each bus signal and ground.
Some high speed bus designs use diodes at each end of the bus between the signal and then power rails to eliminate
under and overshoot although I have not seen this done on ISA cards.
At least one manufacturer provides an ISA card that contains nothing but the terminating network. See attached for
example of this and other bits and pieces that may be of interest.
The noise immunity of the ISA bus will be lowered if the supply voltage is low. All nearby high speed leads such as
the VGA adapter etc should be kept short or screened, and the current setup has much room for improvement.
It should be noted that there is limited space within the current ISA backplane cage, it would be possible to install
only one more ISA or PC104 card.
Mike Rose 17/5/99