ast —
multiplexing serial communications interface
ast0 at isa? port 0x1a0 irq 5
com2 at ast? slave ?
com3 at ast? slave ?
com4 at ast? slave ?
com5 at ast? slave ?
The ast driver provides support for boards that
  multiplex together up to four EIA RS-232C (CCITT V.28) communications
  interfaces. Apparently the original maker of hardware using this multiplexing
  protocol was AST.
Each ast device is the master device for
    up to four com devices. The kernel configuration
    specifies these com devices as slave devices of the
    ast device, as shown in the synopsis. The slave ID
    given for each com device determines which bit in
    the interrupt multiplexing register is tested to find interrupts for that
    device. The port specification for the ast device is
    used to compute the base addresses for the com
    subdevices and the port for the interrupt multiplexing register.
The ast driver was written by Roland McGrath and placed
  into the public domain.