IO: Serial Port I/O Handler. Abstract: The IO handler is a renamed revision of the SP handler, the major improvement being the use of a high-speed ring buffer. This attempts to emulate the FIFO buffer to be found on a multiplexor. The rename is to avoid conflict with the RT11 V5.1 spooler handler. The handler provides for full duplex I/O on up to eight serial interface ports at one time. In particular, single character transfers are possible, without the overhead of multiple I/O requests, and without the need for privileged mapping of the user program (which would provide access to the I/O page). Also important is the provision of high-speed input to a TSX-Plus program. Two modes of operation are possible. The output-only mode, which uses the DC1/DC3 (X-ON/X-OFF) protocol, is invoked by a keyboard COPY command, a FORTRAN WRITE statement, or a .WRITE programmed request in MACRO. The full duplex mode, for which there are various protocol options, is available only through the .SPFUN programmed request in MACRO. At any one time, up to eight serial ports may be active, performing either output or full duplex I/O, any one job may be communicating with more than one serial port, and a number of different jobs may be using the handler. This multiple job capability is particularly useful in RT11XM and TSX-Plus applications. A user manual and installation instructions are included.