Blxxdy hell !
You are screwed.
The bios settings on the motherboard are read by the A2100 application software and if they are not spot on it will be just like the board not being there.
You cant just plug a monitor/psu/keyboard into that motherboard and change settings etc without it screwing things up. you need to download new settings directly into the bios, which is not something you and I can do.
Even a dead battery will screw up either one of your pc motherboards in your a2100 control.
the little 2 wire lead that goes from your motherboard thats on the far right hand side of the a2100 rack that plugs into the bridge board is very important, make sure its plugged in.
The bridge board is might expensive
your best option for spares might be Siemens in the USA as they took over vickers.
I don't know what your machine is like but an arrow 500 with A2100 to a UK based dealer is only worth £4,000.
You could easily spend that fixing yours.
Their was a guy on ebay sellying A2100 hard drives with pre-installed firmware on, until you have a working hard drive I wouldn't want to trust what the error messages say when the machine boots up.
If you want to break it up for spares let me know !