# Using serial interfaces

### Connecting a terminal

You can easily connect a terminal to the serial interface of the simulated microcontroller. This terminal is just a file so it can be anything which is represented as a file. It can be a real serial line of the computer:

    $ s51 -s/dev/ttyS1

Of course you must use the actual device name of your operating system. Device name `ttyS1` above is used in Linux systems. Your system can use other names.

You can use a terminal of your system. It can be a virtual console if your system provides such as Linux does for example. On X Windows you can use xterm windows as terminals, one for running the simulator and one as a terminal on CPU's serial line. Here is a sample how to do this:

1. Prepare the terminal window which will be connected to the serial line:

   - Check the device name which represents the terminal:

         $ tty
         /dev/ttyp1

   - Disconnect the shell from the terminal. Usualy I use the tail command and any existing text file:

         $ tail -f $HOME/.profile

2. Run the simulator in the other window:

       $ s51 -s/dev/ttyp1 program.hex

   Use the output of the tty command above as the parameter of the `-s` option.

Every character sent out by the simulated program appears in the "terminal" window and every charater you type in there will be received by the simulated controller's serial line.

### Connecting two instances of simulator

Executing two instances of the simulator, serial lines of two simulators (micros) can be connected together so they can talk to each other over their serial interface. It is because you can specify separate files for serial input and output. For example you run two simulators "1" and "2", here is the sample how to connect them:

1. Make two FIFOs to represent physical wires in serial cable connecting two micros:

       $ mkfifo 1-2 2-1 # 1-2: 1->2  and 2-1: 2->1

2. Start two simulators and specify the FIFOs as input and output of serial interface:

       term1 $ # start sim "1"
       term1 $ s51 -Sin=2-1,out=1-2 program_1_.hex
       term2 $ # start sim "2"
       term2 $ s51 -Sout=2-1,in=1-2 program_2_.hex

   Because opening a pipe blocks the program until other direction is opened, the order of arguments above is important!

3. Debug programs as usual.

Using the most usefull unix commands cat and tee and just some more FIFOs you can monitor serial communication, here is a sample:

1. Make some FIFOs to use between simulators and tee "monitors":

       $ mkfifo 1_tee tee_2 2_tee tee_2

2. Run monitoring programs (in two xterms for example):

       xterm1 $ cat 1_tee|tee /dev/tty >tee_2 # monitor 1->2
       xterm2 $ cat 2_tee|tee /dev/tty >tee_1 # monitor 2->1

3. Now you can start simulators (on two other terminals:)

       xterm3 $ s51 -Sin=tee_1,out=1_tee program_1_.hex
       xterm4 $ s51 -Sin=tee_2,out=2_tee program_2_.hex

4. Start your apps and listen what they are talking about.