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Device drivers for asynchronous serial lines


The  COHERENT system  has  drivers for  four asynchronous  serial
lines, ccoomm11 through ccoomm44.

A  serial  line  can   be  opened  into  any  of  four  different
``flavors'', as follows:


     ccoomm?ll     Interrupt driven, local mode (no modem control)
     ccoomm?rr     Interrupt driven, remote mode (modem control)
     ccoomm?ppll    Polled, local mode (no modem control)
     ccoomm?pprr    Polled, remote mode (modem control)


``Local mode''  means that the line will  have a terminal plugged
into  it, to  directly  access the  computer.  ``Modem  control''
means that  the line  will have a  modem plugged into  it.  Modem
control  is  enabled on  a  serial line  by  resetting the  modem
control bit  (bit 7)  in the minor  number for the  device.  This
allows  the system  to generate  a hangup  signal when  the modem
indicates loss of  carrier by dropping DCD (Data Carrier Detect).
A  modem  line should  always  have  its DSR,  DCD  and CTS  pins
connected.   If  left  hanging,  spurious transitions  can  cause
severe  system thrashing.   To disable modem  control on  a given
serial line, use the minor device which has the modem control bit
set (bit 7).  An ooppeenn to  a modem-control line will block until a
carrier is detected (DCD goes true).

``Interrupt mode'' means  that the port can generate an interrupt
to attract the  attention of the COHERENT system; ``polled mode''
means that  the port  cannot generate  an interrupt, but  must be
checked (or ``polled'')  constantly by the COHERENT system to see
if activity has occurred on it.

The  COHERENT system  uses two  device  drivers to  manage serial
lines: one  driver manages COM1  and COM3, and  the other manages
COM2 and  COM4.  Due to  limitations in the design  of the ports,
you can enable  interrupts on either COM1 or COM3  (or on COM2 or
COM4),  but   not  both.    If  you   wish  to  use   both  ports
simultaneously, one must be  run in polled mode.  For example, if
you wish to  open all four serial lines, you  can open two of the
lines  in interrupt  mode: you  can open either  COM1 or  COM3 in
interrupt mode, and you can open either COM2 or COM4 in interrupt
mode.  The other two lines must be opened in polled mode.

Opening a  device in polled mode consumes  many CPU cycles, based
upon the speed of  the highest baud rate requested.  For example,
on a  20 MHz 80386-based machine, polling  at 9600-baud was found
to consume about 15% of the CPU time.  As only one device can use
the interrupt  line at  any given time,  the best approach  is to
make the  high-speed line of  the pair interrupt  driven and open
the  low-speed  or  less-frequently  used  line in  polled  mode.


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However, if you enable a polled line for logins, the port is open
and will  be polled as  long as the port  remains open (enabled).
Thus, even if a port is  not in use, the fact that it has a ggeettttyy
on it  consumes CPU cycles.  As  a rule of thumb,  try and open a
port in  interrupt mode.  If you cannot,  use the polled version.
Also note that use of any of the four serial ports in polled mode
prevents  other polled  serial  device drivers,  such  as the  hhss
generic multi-port  polled serial driver, from  being used at the
same time.

If you intend to use a modem on your serial port, you must insure
that the DCD signal from  the modem actually _f_o_l_l_o_w_s the state of
carrier detect.   Some modems allow the user  to ``strap'' or set
the  DCD  signal so  that  it is  always  asserted (true).   This
incorrect setup  will cause COHERENT  to think that  the modem is
``connected''  to a  remote  modem, even  when there  is no  such
connection.

In addition, if you wish  to allow remote logins to your COHERENT
system via  your modem, you  must insure that the  modem does nnoott
echo any  commands or status information.  Failure  to do so will
result  in severe  system  thrashing due  to the  ggeettttyy or  llooggiinn
processes endlessly ``talking'' to your modem.

***** Changing Default Port Speeds *****

Serial lines ccoomm11 through  ccoomm44 default to 9600 baud when opened.
This default  speed can  be permanently  changed on a  "per port"
basis by changing the value of driver variables CC11BBAAUUDD_, CC22BBAAUUDD_,
CC33BBAAUUDD_ or CC44BBAAUUDD_. The list of acceptible values can be found in
header file <ssggttttyy.hh> and range from 1, corresponding to 50 baud,
up to 17, which corresponds to 19,200 baud.  For a table of legal
baud rates, see the Lexicon entry for ssggttttyy.hh.

To  change  the  default value  for  a  port,  you  must use  the
/ccoonnff/ppaattcchh command.   For example,  to change the  default speed
for port  ccoomm22 to  2400 baud,  enter the following  command while
running as the superuser:


          /conf/patch /drv/al1 C2BAUD_=12


The change will not take effect until the next time that you boot
your system.

***** See Also *****

ccoomm11, ccoomm22, ccoomm33, ccoomm44, ddeevviiccee ddrriivveerrss

***** Diagnostics *****

An  attempt to  open a  non-existent  device will  generate error
messages.  This can occur if hardware is absent or not turned on.



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***** Notes *****

The ccoomm*  series of devices  are not compatible  with the iiooccttll()
parameters defined in  header file <tteerrmmiioo.hh>. Be sure to include
header file  <ssggttttyy.hh> if you  wish to perform  terminal specific
iiooccttll() calls.

In the  current version of these  drivers, the following sequence
of steps results in a panic:


          enable com4pl
          enable com3pl
          disable com4pl
          kill kill <al1 driver process id>


The key is that  the driver containing the polling routine cannot
be unloaded if the other driver is still polling.

Note, too, that if any ccoomm device driver is used in polling mode,
the hhss driver cannot be used, and vice versa.



































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