Serial Port I/O

Until about 20 years ago, most desktop and portable computers had serial and parallel ports built in.  Those ports were great for controlling and testing home brew gadgets.  Modern machines are blessed/cursed with USB ports, which are just getting more and more complex.

The result is that if you want to do anything at all in your Underground Lab or Rooftop Radio Shack, you need an Arduino,  a Raspberry or a Beaglebone embedded computer.

However, every respectable engineer has two or three FTDI USB to RS232 serial adaptors lying in a drawer (to control his Raspberry or Beaglebone).  These adaptors are great, since they effectively insulate your target system from your expensive computer, therefore whatever goes wrong on the far end, is unlikely to fry your machine and they are delightfully hackable.

I prefer the dongles made by SerialComm.  They are cheap and both RS232 and RS422/485 are available.

You can also get ones from Sparkfun that are even more hackable, or you can crack the case and remove the line driver chip from a regular off the shelf one if you are desperate for TTL I/O lines.

The secret Open Sauce is the libftdi project, which is available for Linux and Mac systems. Documentation and examples are here.  The online files are the latest and greatest.  Therefore it may be best to work with the header file on your machine which is /usr/include/ftdi.h since your installed version of the library may be older and some functions may be deprecated or missing.

Simple Bitbanging with libftdi

Here is a tiny little C program for an older version of libftdi, that will toggle the Tx, DTR and CTS lines on a USB RS232 adaptor.  Note that you have to run it as root (or join some or other USB group), otherwise the USB device will not open.

/* File flasher.c */
/* libftdi API Example LED Flasher */

#include <stdio.h>
#include <ftdi.h>

/* RS232 DE9 pins */
#define DCD 0x40 /* 1 in */
#define RX  0x02 /* 2 in */
#define TX  0x01 /* 3 out */
#define DTR 0x10 /* 4 out */
#define DSR 0x20 /* 6 in */
#define RTS 0x04 /* 7 out */
#define CTS 0x08 /* 8 in */
#define RI  0x80 /* 9 in */
/* 5 GND */


int main()
{
    unsigned char data = 0;
    unsigned char pins = TX | DTR | RTS;
    struct ftdi_context context;

    /* Initialize libftdi */
    ftdi_init(&context);

    /* Open FTDI dongle using FT232RL vendor & product IDs */
    if(ftdi_usb_open(&context, 0x0403, 0x6001) < 0) 
    {
        puts("ERROR: ftdi_usb_open()");
        return 1;
    }

    /* Set bitbang mode on the RS232 output pins */
    ftdi_enable_bitbang(&context, pins);

    /* Forever */
    for(;;) 
    {
        data ^= pins;
        ftdi_write_data(&context, &data, 1);
        sleep(1);
    }
}

Improved Bitbanging with libftdi

Here is a more advanced program with proper error checking.  Note that you have to run it as root (or join some or other USB group), otherwise the USB device will not open.

/* File flasher.c */
/* libftdi.so API Example LED Flasher */
/* Copyright reserved Herman Oosthuysen, 2015 */
/* License: GPL version 2 or later */
/* Use at your own peril */

#include <stdio.h>
#include <stdlib.h>
#include <ftdi.h>

/* RS232 DE9 pins */
#define DCD 0x40 /* 1 in */
#define RX  0x02 /* 2 in */
#define TX  0x01 /* 3 out */
#define DTR 0x10 /* 4 out */
#define DSR 0x20 /* 6 in */
#define RTS 0x04 /* 7 out */
#define CTS 0x08 /* 8 in */
#define RI  0x80 /* 9 in */
/* 5 GND */


int main()
{
    int i;
    int ret;
    unsigned char data = 0;
    unsigned char outputs = TX | DTR | RTS;
    struct ftdi_context *ftdi;

    /* Initialize libftdi */
    printf("FTDI Initialize\n");
    ftdi = ftdi_new();
    if (ftdi == NULL)
    {
        fprintf(stderr, "ERROR: ftdi_new()\n");
        return EXIT_FAILURE;
    }

    ret = ftdi_init(ftdi);
    if (ret < 0)
    {
        fprintf(stderr, "ERROR: ftdi_init() = %d\n", ret);
        ftdi_free(ftdi);
        return EXIT_FAILURE;
    }

    /* Open FTDI dongle using FT232RL vendor & product IDs */
    /* Plug the device in and run 'dmesg' to see these codes */
    printf("FTDI USB Open\n");
    ret = ftdi_usb_open(ftdi, 0x0403, 0x6001);
    if (ret < 0)
    {
        fprintf(stderr, "ERROR: ftdi_usb_open() = %d\n", ret);
        ftdi_free(ftdi);
        return EXIT_FAILURE;
    }

    /* Set bitbang mode on the RS232 output pins */
    /* pins: Output = 1, Input = 0 */
    printf("FTDI set Bitbang Mode\n");
    ret = ftdi_set_bitmode(ftdi, outputs, BITMODE_BITBANG);
    if (ret < 0)
    {
        fprintf(stderr, "ERROR: ftdi_set_bitmode() = %d\n", ret);
        ftdi_free(ftdi);
        return EXIT_FAILURE;
    }

    /* Flash for a little while */
    /* Note: Use ftdi_read_pins() to read data directly */
    printf("FTDI Flashing...\n");
    for(i = 0; i < 10; i++)
    {
        printf("%d\r", i);
        data ^= outputs;
        ret = ftdi_write_data(ftdi, &data, sizeof(data));
        if (ret < 0)
        {
            ftdi_free(ftdi);
            fprintf(stderr, "ERROR: ftdi_write_data() = %d\n", ret);
            return EXIT_FAILURE;
        }
        usleep(500000);
    }

    /* Done */
    printf("\nDone\n");
    ret = ftdi_usb_close(ftdi);
    if (ret)
    {
        ftdi_free(ftdi);
        fprintf(stderr, "ERROR: ftdi_usb_close() = %d\n", ret);
        return EXIT_FAILURE;
    }

    ftdi_free(ftdi);
    return EXIT_SUCCESS;
}

CBUS Discretes

In addition to the 8 lines used for RS232, there are 4 more, which are used to control the LEDs and RS485 drivers.  These lines can be controlled in a similar fashion using BITMODE_CBUS.  It is not clear whether one can interleave the two bitbashing modes in order to control all 12 lines at the same time and whether the lines will glitch if one does. See this example.

I also read that one can use the CBUS bitbashing concurrently with the normal serial mode, to provide a UART plus 4 discretes, but I have not tried it.

Compile and Test

Assuming that your Linux machine is configured properly with GCC, compile it thus:
$ gcc -o flasher flasher.c -lftdi
$ chmod 754 flasher

Now stick a LED with a 1k resistor in series onto the RS232 adaptor Tx and Gnd pins to see how it works and run it:
$ ./flasher

Install and Configuration of libftdi

If you don't have a GCC and libftdi configured system yet, then assuming that you have Fedora Linux:
$ su -
# yum update

# yum install kernel-headers

# yum groupinstall "Development Tools" "Development Libraries"
# yum install libftdi libftdi-devel

Now you can control the world!

Cmake

Also see the next post on Makefiles. 

Serial Port Tips

www.aeronetworks.ca/2015/01/serial-port-io.html
www.aeronetworks.ca/2014/10/serial-ports-revisited.html
www.aeronetworks.ca/2014/01/crcs-and-serial-ports.html
www.aeronetworks.ca/2013/10/serial-port-tricks.html
www.aeronetworks.ca/2013/05/usb-serial-device-with-unknown-ids.html
www.aeronetworks.ca/2015/10/reading-and-parsing-data-from-serial.html
www.aeronetworks.ca/2013/05/compile-moxa-serial-widget-device.html

La voila,

Herman