The serial multiplexer

The eLua serial multiplexer

(v0.8 and above) The serial multiplexer is an optional eLua component that allows using several serial ports over a single physical serial link connection. It can be a very convenient feature, since some eLua components (such as the RPC mechanism or the remote file system) need a serial link to the PC and it’s very incovenient (or even impossible sometimes) to connect the eLua board to the PC with 2 or more serial cables. A common use scenario for the serial multiplexer is to have the eLua shell and RFS running over a single physical connection to the PC, a very convenient method that will probably appeal to most eLua users.

How does it work?

On the eLua side a number of virtual serial ports are defined. The eLua code can use these ports just like they would use a physical port, simply by specifing a virtual port ID instead of a physical port ID. On the PC side, two things are needed:

  • a program that can (de)multiplex UART requests from the eLua board. It is called mux and it is part of the standard eLua distribution.

  • a mechanism for creating and using "virtual serial ports" on the PC itself. These are actually pairs of virtual UARTs that are connected internally (inside the OS) via a null-modem cable mechanism, which means that when you type something on one port you can see its output on its pair (and the other way around).

See below for details on how to use virtual serial ports in Linux and Windows.

Virtual serial ports in Linux

Linux already supports the virtual serial port mechanism described above via the standard UNIX pseudo terminals, see here for details. Note that the multiplexer supports only BSD PTYs (dev/ttypx) and not Unix98 PTYs (/dev/ptmx). They are quite standard (although they are becoming obsolete in some Linux distributions) so you shouldn’t have any problems with them. A quick check for BSD PTYs is to look in your /dev directory for ttypx and ptypx files. If they aren’t there, you need to enable support for BSD PTYs in your system. How to do this is OS dependent and beyond the scope of this tutorial, so google is your friend. For an example on how to enable them in Ubuntu check this link.

Once they are enabled you can get a quick feel of how their work. /dev/ttypx and /dev/ptypx are paired by default, so any transmit/receive at one end is mirrored at the other end. A simple experiment:

  • start two shell sessions

  • execute screen /dev/ttyp0 115200 in one of the shells

  • execute screen /dev/ptyp0 115200 in the other shell

Now everything you type in one of the shell sessions should be visible in the other one.

Virtual serial ports in Windows

Windows doesn’t have out-of-the-box support for virtual serial ports, but fortunately there’s an extremely nice open source program that does exactly what eLua needs. It is called com0com. Download it, install it, then open the com0com serial port manager to create your virtual serial port pairs. Then give it a little spin to get used to how it works. Supposing that you created COM10 and COM11 as a virtual serial port pair, try this:

  • start your terminal emulator program. My preferred terminal emulator program in Windows is TeraTerm, but you can use any emulator you want. Open COM10 at baud 115200.

  • start another instance of the terminal emulator, but this time open COM11 at baud 115200.

Now everything you type in one of the terminal emulators should be visible in the other one.

Note com0com can create pairs of serial ports with unusual names, for example CNCA1 and CNCB1. While they work fine with eLua's serial multiplexer, they might not work equally well with terminal emulator programs, so you’re advised to stick with standard port names (COMx).

Enabling the serial multiplexer in Lua

This is the easy part, just follow the instruction in the building page. You need to define the following macros for the serial multiplexer:

Option Meaning


Enable serial multiplexer support in eLua.


The ID of the physical UART interface used by the serial multiplexer.


Communication speed of the multiplexer UART interface.


Flow control type on the physical serial multiplexer interface, see here for details. If not specified it defaults to 'no flow control'.


The number of virtual UART interfaces. This number can’t be higher than 8.


An array of SERMUX_NUM_VUART integers that specify the buffer sizes for the virtual UART interfaces. Note that a virtual UART MUST have a buffer associated with it. The sizes are specified as BUF_SIZE_xxx constants defined in inc/buf.h

As a simple example, let’s change the configuration of an eLua board that uses UART 0 as its console UART to use a serial multiplexer with 2 ports (one for RFS and the other one for console) over UART 0. The original configuration (in src/platform/<platform>/platform_conf.h) will look like this:

#define CON_UART_ID           0
#define CON_UART_SPEED        115200

The new configuration should be similar to the one below:

#include "sermux.h"                                             // for virtual uart IDs
#include "buf.h"                                                // for buffer sizes

#define BUILD_SERMUX                                            // enable serial multiplexer support
#define CON_UART_ID         ( SERMUX_SERVICE_ID_FIRST + 1 )     // console runs on the second virtual UART
#define CON_BUF_SIZE        BUF_SIZE_128                        // size of console UART buffer, cannot be 0
#define RFS_UART_ID         ( SERMUX_SERVICE_ID_FIRST )         // RFS runs on the first virtual UART
#define RFS_BUFFER_SIZE     BUF_SIZE_512                        // size of the RFS UART buffer, cannot be 0
// Serial multiplexer data
#define SERMUX_PHYS_ID      0                                   // multiplexer runs on UART 0
#define SERMUX_PHYS_SPEED   115200                              // multiplexer runs at 115200 baud
#define SERMUX_NUM_VUART    2                                   // multiplexer creates 2 virtual UARTs
#define SERMUX_BUFFER_SIZES { RFS_BUFFER_SIZE, CON_BUF_SIZE }   // buffer sizes for the virtual UARTs

This sequence of macro definitions will enable serial multiplexer support in eLua and will allow the code to use two virtual UARTs. The virtual UART IDs start at SERMUX_SERVICE_ID_FIRST. A maximum of 8 virtual UARTs are supported by the system. In this particular example the first virtual UART is assigned to RFS and the second one to the system console.

Important when using the RFS in a virtual UART configuration, remember that RFS_UART_ID must be the first virtual UART ID in the system (SERMUX_SERVICE_ID_FIRST). Otherwise, the serial multiplexer will NOT work properly in rfsmux mode. Check here for details, and here for more details about the RFS.

Serial multiplexer on the PC side

The serial multiplexer needs a program running on the PC side, the mux server. This is the program that implements the actual serial port multiplexing (eLua connects to this program via the SERMUX_PHYS_ID UART). The mux server was succesfully built and tested under Windows 7 (32 bit) and Linux (Ubuntu 64 bit). To build it you need:

  • a build environment. This means gcc under Linux (see for example here for instruction on how to install a toolchain in Ubuntu Linux) and a gcc-based development environment under Windows. How to install a gcc-based development environment under Windows is beyond the scope of this tutorial, but check here and here for possible solution (the mux server was compiled and tested under Win32 using the second soluation, the tdm gcc compiler).

  • scons. See here for details on how to install scons on Windows, or here for details on how to install scons on Linux.

If the above requirements are met, building the mux server is a simple matter of invoking this command from the eLua source tree base directory:

scons -f

After this you should end up with a mux.exe file in Windows, or a mux file in Linux. Running it without arguments prints the usage help:

Usage: mux <mode> <transport> <vcom1> [<vcom2>] ... [<vcomn>] [-v]
    'mux':                 serial multiplexer mode
    'rfsmux:<directory>:   combined RFS and multiplexer mode.
  transport: '<port>,<baud>,<flow> ('flow' specifies the flow control type and can be 'none' or 'rtscts').
  vcom1, ..., vcomn: multiplexer serial ports.  Use '-v' for verbose output.

Using the multiplexer in "mux" mode

This is the basic use scenario for the serial multiplexer. Im this mode mux will simply multiplex the serial connection with the eLua board with a number of virtual serial ports in the system.

This is best understood by an example. We’ll ask mux to multiplex two serial ports for us, these will be dev/ptyp0 (and its pair /dev/ttyp0) and /dev/ptyp1 (and its pair /dev/ttyp1) in Linux and COM10 (and its pair COM11) and COM20 (and its pair COM21) in Windows. The physical UART is /dev/ttyUSB0 in Linux and COM5 in Windows. So, to run it in Linux, execute this:

./mux.exe mux /dev/ttyUSB0,115200,rtscts /dev/ptyp0 /dev/ptyp1

In Windows:

mux mux com5,115200,rtscts com10 com20

And now let’s put this to good use. Remember that in the previous paragraph we built an eLua configuration that assigned RFS to the first virtual UART (which corresponds to the vcom1 argument of mux) and the console to the second one (which corresponds to the vcom2 argument of mux). To make this happen on the PC, we start the respective services on the UART pairs of the vcomx arguments of mux. In Linux:

./rfs_server ser:/dev/ttyp0,115200,none /home/user/work/fs
screen /dev/ttyp1 115200

In Windows:

rfs_server ser:com11,115200,none c:\elua\fs
(also run TeraTerm or your preffered terminal emulator on port COM21 at 115200 baud)

Reset your eLua board, and you’re ready to go! mux will send all the RFS requests to /dev/ptyp0 (or COM10) which in turn gets automatically redirected to /dev/ttyp0 (or COM11) and will redirect all console I/O to /dev/ptyp1 (or COM20) which in turn gets automatically redirected to /dev/ptyp1 (or COM21).

Although this works, there is a simpler, more convenient way to do it if RFS support is needed: use the rfsmux mode instead.

Using the multiplexer in "rfsmux" mode

To make things easier for RFS users mux includes a special operation mode (rfsmux) that automatically "talks" to the RFS server. The user doesn’t need to manually assign a virtual UART for the RFS server and start the RFS server. In this mode mux uses a special internal communication channel to the RFS server (that doesn’t require a virtual UART). A strict requirement for this mode is that eLua must be configured to assign the first virtual UART ID (SERMUX_SERVICE_ID_FIRST) to the RFS server (RFS_UART_ID). If this doesn’t happen, rfsmux mode will not work anymore (but note that you still can use the mux mode described above with this setup).

This is best understood by an example. We’ll ask mux to share a directory and to multiplex one single serial port for us (we’ll use it for the console), this will be dev/ptyp0 (and its pair /dev/ttyp0) in Linux and COM10 (and its pair COM11) in Windows. The physical UART is /dev/ttyUSB0 in Linux and COM5 in Windows. So, to run it in Linux, execute this:

./mux.exe rfsmux:/home/user/work/fs /dev/ttyUSB0,115200,rtscts /dev/ptyp0

In Windows:

mux rfsmux:c:\elua\fs com5,115200,rtscts com10

All that’s left now is to run the terminal emulator. In Linux:

screen /dev/ttyp0 115200

In Windows simply start TeraTerm or your preffered terminal emulator on port COM11 at 115200 baud.

Reset your eLua board, and you’re ready to go! mux will send all the RFS requests to the RFS server via its internal channel and will redirect all console I/O to /dev/ptyp0 (or COM10) which in turn gets automatically redirected to /dev/ttyp0 (or COM11).


Some things you should consider when using the serial multiplexer:

  • the code is still in beta. It works well most of the time, but sometimes it simply crashes. If this happens, please consider submitting a bug report.

  • if your eLua board has two hardware serial ports that you can use and the PC has also two free serial ports, consider disabling the serial multiplexer completely. Run RFS on a hardware port and the system console on the other hardware port instead. This is both more realiable and more efficient.

  • to avoid problems with the serial multiplexer use this sequence to start it:

    1. start mux

    2. start the terminal emulator

    3. reset the eLua board

  • using hardware flow control is strongly encouraged. To do this:

    1. make sure that your eLua board has support for hardware flow control (see here for details).

    2. specify the correct SERMUX_FLOW_TYPE value at build time (it should be PLATFORM_UART_FLOW_RTS | PLATFORM_UART_FLOW_CTS).

    3. make sure that the serial cable connecting the PC and the eLua board also supports flow control. Some simple serial connection cables have only the RX, TX and GND wires. RTS/CTS flow control requires at least RX, TX, RTS, CTS and GND wires arranged in a null-modem configuration.

    4. start mux specifying rtscts as part of the <transport> parameter (see above).

  • the serial multiplexer "protocol" is an extremely simple one, it doesn’t make provisions for error correction or detection, and it might loose synchronization if there are errors on the serial line. So, if it starts behaving abnormally, you might want to restart mux (and rfs_server if you’re running it with mux) and reset your eLua board.

  • some serial ports built around USB to RS232 adapters seem to confuse mux sometimes. If mux won’t work after you tried all the above instructions, or if mux terminates unexpectedly, unplugging and plugging the USB cable of the RS232 adapter and restarting mux will most likely solve your problem.

  • if you get an "Error on select, aborting program" error from mux, keep in mind that this is normal if you run a terminal emulator (screen) under Linux on a virtual UART and then close it (by exiting screen). However, it is not normal if it happens under other circumstances in Linux, or if it happens in Windows. In these cases, please consider submitting a bug report.

  • if the serial multiplexer is enabled on the eLua board it’s not possible to use the board with a regular terminal emulator anymore (without running mux), although it might appear so. eLua will send some output to the terminal emulator, but it won’t be able to accept any input from it.

  • if you find a bug in mux and wish to report it, try to reproduce the problem again, but this time run mux with -v (verbose). The resulting logs may help us identify the problem.