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INTRODUCTION

  Clipper Tools One serial interface functions can be used to control two
  interfaces simultaneously.  Characters arriving at a serial port are sent
  via an interrupt service to a ring buffer.  A ring buffer is initialized
  for each port.  The Clipper program can then be used to determine the
  number of available characters in a buffer and read any number into the
  program.  A software or hardware handshake can, if necessary be used.
  It is possible for all arguments of the interface, such as baud rate,
  parity, data lengths and stop bits to be easily changed.  It is possible
  to change the baud rate for an interface without "closing" it and there-
  fore possible to change the transmission speed without losing the contents
  of the buffer, or even disconnecting an existing connection (DTR signal).

  All important control signals such as CD (Carrier Detect), DTR (Data
  Terminal Ready), etc., can be changed or read.  In order to simplify the
  programming, a separate function for each status flag exists, enabling
  the facility for directly reading and writing corresponding registers
  for all status and control signals.

  All serial interface functions bypass the BIOS and other DOS calls, and
  talk directly to the hardware.  As a result of this only hardware which
  is 100 percent compatible with IBM personal computers can be used.
  Clipper Tools One will determine the I/O addresses of the two serial
  interfaces from the BIOS data area.

  In order to guarantee that everything functions correctly, both interfaces
  must either be equipped with 8250 serial controller chips or compatible
  16450.  The interface must then be fitted with a quartz crystal with a
  frequency of 1.843 megahertz, which is standard for PC interfaces.  Please
  refer to the appropriate technical descriptions for the technical details
  of the interfaces and registers, i.e., the IBM Technical Reference Manual.

  Warning: The interrupt vector of the interface concerned will be changed
  as soon as it has been opened with COM_INIT().  Open interfaces must either
  be closed before exiting the program, or the expanded driver CTOOLD87.OBJ
  (described elsewhere) which saves all interrupt vectors from 0 to 127 and
  restores them when a program exits normally, should be used.  Please read
  the "Expanded Driver" chapter for more details.