SERIAL COMMUNICATIONS

On PCs with high-quality buffered UARTs, Kermit can be used at speeds up to
57600 bps under DOS (under Windows or DesqView, the maximum speed is probably
lower).	 115200 bps works only with a very short shielded cable, and the async
adapters of the two machines in perfect tune.  Some VAX serial port interfaces
are out of tolerance at 19,200 bps and faster.	Some of the new high-speed PCs
(486s like, for example, the IBM PS/VP or the Gateway 2000) come with
unbuffered UARTs.  Despite the speed of the CPUs, these new machines perform
serial communications less reliably than older machines that run at 1/4 the
speed or less, but that have buffered UARTs (like the PS/2-70).

MS-DOS Kermit will tell you whether you have a buffered UART:

  SET PORT 2
  SHOW COMMUNICATIONS
  ...
  COM1 Address: Port \xf38, IRQ 4, 16550A UART FIFO

16550A UART means you have the good kind, FIFO means it has a "First-In
First-Out" buffer.  If Kermit does not say you have a 1655A UART FIFO, and you
are suffering from performance or data loss problems, replace it (if
possible!)  with a 16550A.

If your speed is set too high, the symptom might be lost or garbled characters
or graphics images on the screen, or data-overrun beeps.  Normally flow control
prevents these problems so use it if you can.  Printing while in CONNECT mode
needs flow control to be active.

MS-DOS Kermit does not monitor the communication line for carrier loss during
terminal emulation or file transfer.  You can SHOW MODEM or SHOW COMMUNICATIONS
to inquire about CD, DSR, and CTS.  The script command WAIT permits sensing
carrier presence in a script program.

The SHOW MODEM and WAIT commands work right only if your modem or other
communication device is configured to raise and lower the DSR, CTS, and CD
signals appropriately, and the cable that connects your PC to the modem passes
these signals through.	For some modems, the factory setting is to always keep
CD on, even if there is no connection.	Consult your modem manual.

For serial devices, the HANGUP command (and Ctrl-]H in CONNECT mode) works
only if the cable that connects your PC to the communication device passes the
DTR signal through, and if the communication device itself is configured to
hang up or otherwise terminate the connection when the DTR signal is lowered
by the PC.  For some modems, the factory setting is to ignore DTR transitions.
Consult your modem manual (for example, tell a Hayes modem to "AT&D2").	 NOTE:
in version 3.11, the HANGUP command turned off DTR for about 1/2 second and
then turned it back on again.  In version 3.12 and later, the HANGUP command
turns off DTR until the next time you issue a command that accesses the serial
port.  If you can't make the HANGUP command work, define a macro to send a
hangup command to the modem, for example (for Hayes modems):

  DEFINE MYHANGUP PAUSE 1, OUTPUT +++, PAUSE 1, OUTPUT ATH0\13

and then use this macro in place of the regular HANGUP command.

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