sit-link implements a virtual serial link between two simulations
by passing packets of serial data over TCP sockets. The TCP 
connection can be over the internet, so sit-link allows interoperability
testing without unnecessarily exposing a company's intellectual
property.

sit-link consists of 2 main components: an RTL module that is instantiated
into the simulation and represents the connections to the serial link; 
and an application that uses named pipes (FIFOs) to pass data to and from 
the simulation and a TCP socket to pass data to and from the remote
simulation.  These components are identical on either side of the serial
link, except that one side is setup as listening for the connection to
open, and the other side initiates the connection.

sit_ver_x4.v is a verilog module for a x4 connection. It has a 4 bit input,
tdata, and a 4 bit output, rdata.  Unless a parameter is defined or a
command line option is set, the named pipes it uses will be sit_pipei
and sit_pipeo.

sit_cli is an executable that implements the connection.  Invoke sit_cli
with either -L (for listen) or the name of the remote node on which another
sit_cli process is listening.


For example (assuming sit_cli is in your $path):
 On node 1:
   % cd <simulation directory>
   % mkfifo sit_pipei sit_pipeo
   % sit_cli -L &
   % <invoke simulation>

 On node 2:
   % cd <other simulation directory>
   % mkfifo sit_pipei sit_pipeo
   % sit_cli node1 &
   % <invoke simulation>

(Note: the mkfifo command is only needed once - once it's run, the named
pipes remain in the directory even after they are used).


An extremely simple and brief example application is included as x4_test.v
and would be run under VCS as so:

 On node 1:
   % mkfifo sit_pipei sit_pipeo
   % sit_cli -L &
   % vcs -R x4_test.v sit_ver_x4.v

 On node 2:
   % mkfifo sit_pipei sit_pipeo
   % sit_cli node1 &
   % vcs -R x4_test.v sit_ver_x4.v


Important note:
  sit-link simply samples the serial data every 400ps for replication to the
remote simulation, so any variation of bit times should be disabled for 
simulations using sit-link.

Notes regarding different simulators:
  When using Modelsim, it is necessary to run sit_cli first, and that
sit_cli makes a network connection before Modelsim runs.  It may be useful
to use the -c option of sit_cli to start the simulation.


sit_cli options:
  sit_cli now supports several options:

    -pipeloop - this puts sit_cli into a local loopback mode in which
      no network conneciton is made, and any data from the pipe is looped
      back to the pipe. This is useful for testing.

    -sockloop - this is a second loopback mode, in which a network connection
      is made, but any data from the network connection is looped back, and
      the named pipes are not opened or used.

    -rerun - this causes sit_cli to be run again (with exactly the same
      parameters) when either the pipes or the socket connection is closed.
      This is particularly useful in combination with the -L (listen) and
      -c (spawn a command) options, allowing an unattended server to be
      arranged.

    -c "command" - after a network connection is made, the command after
     -c will be executed (enclose the command in quotes if it includes
     spaces).  This can be used to delay starting a simulation until a
     network connection is established.

    -n <name> - a substitute name prefix for the named pipes, replacing
     sit_pipei and sit_pipeo. 'i' and 'o' are appended to the specified name.

    -L - listen - instructs sit_cli to listen for an incoming network
     connection on port 52143, instead of establishing a network connection
     to a remote node.

  If a remote nodename is not specified to sit_cli and the -L option is not
used, the default connection will be made to "localhost".