HP 98568A Installation Instruction page 174

HP 98644A Asynchronous Serial Interface
Description and Support
The HP 9S644A Asynchronous Serial Interface connects to a terminal, modem,
serial peripheral, or computer and supports the RS-232C standard. One
interface is required for each device, and each interface must be set to a unique
select code.
HP 98644A Asynchronous Serial Interface
Series 300 HP-UX Operating System Support
on Release with Model
5.0 or later all Series 300 with boot
ROM Rev. A or later
5-78 Adding Interface Cards