The Opto-22 G4PB4R is an output mounting rack compactly accommodates up to four G4 output modules and features channel-to-channel isolation between all field and control circuits. Positive-true and standard negative-true control signals from computer I/O ports may be connected to the racks. Modules are inserted and removed easily and quickly without disturbing the field wiring. The threaded captive hold-down screw on each module securely attaches them to the rack. Barrier strips with screw terminals provide the field, control, and mounting rack power connections.