The Tektronix 021-0374-00 GPIB Decoder is a 7000-series plug-in designed to allow GPIB control of 7A16P or 7A29P programmable amplifiers and/or a 7B90P programmable time base module in a 7854 mainframe. (The manual warns that of all 7000 series mainframes, only the 7854 has adequate +5 V supply capacity.)
The -P type plugins were originally designed for the 7000 series digitizers like the 7912AD that have a built-in GPIB interface.
The 021-0374-00 replicates the functions of that interface, receiving commands from GPIB and translating them to the internal serial format expected by the plugins. It is based on a 6802 microcontroller and a TMS 9914 GPIB interface chip.
The interface must be installed in the left horizontal (A timebase) bay. It addresses left vertical, right vertical and right horizontal plugins through GPIB sub-addresses 1, 2 and 3, respectively.