microvax/vs 2000 expansion base circuitry ?

[Jonathan Stone via cctalk]

I've read that there is circuitry in the expansion base (BA40A?) has circuitry 
.  Does anyone know what the circuitry does?  Is it required for SCSI 
operation?  (I hope not, or I'll have to kludge one up to make use of pk2k SCSI 
boot-roms!)