The 9558M is an external LVD terminator
for terminating an LVD bus on a SCSI system.
Every SCSI subsystem must have a terminator
(terminating resistors) at each end of its
internal and/or external bus. Termination is a unique
approach to data handling, and although it can
appear complicated to the uninitiated, its benefits
are what made SCSI the fastest and most robust communications
interface through the 80's and 90's.
This terminator is mounted on an Ultra-High
Density connector, called a UHD 0.8mm or 80-pin,
so it can plug into an open connector on an 80-pin SCSI
external (female) cable or device.
