Managing fiscal printer from Datecs by browser

I would like to share my experience wth a task I had to do for a project at work. The project is a web application billing software. So it needed to be able to print fiscal notes, bons or whatever. There were several ways of dealing with this stuff – creating a XPCOMM extension for FireFox, using activeX control (supporting just IE) and the java applet way. I examined
the first two approaches and it seemed that they are not supporting all the browsers and environments. Datecs FP 550-05 is the fiscal printer I have to test which has a communication protocol and comes with a ready to use dll file. This supposes the use of activeX – I have no experience with that, no real C experience and it turned out the hardest for me. I skipped this.

Developping XPCOMM extension for FireFox is an interesting stuff to dig in but I had no time for that. I skipped this, too.

The way I went on this was java. I found a very usefull ready to use applet here – JavaScript Serial. There is good explanation on the site but I am a noobie on java. Actually I hate this stuff. I have never liked programming in java. So in addition to the guidelines on the site I would say something more:

• after initializing the applet use setTimeout for about a second before opening the port
• program you peice of software so that your are sure that the port will be closed. If not it cannot be opened afterwards. You have to close the browser.

The rest is just implementing the protocol of the device that you have to support. For the FP 550 the protocol was not very clear about packaging the messages. There was a tool to send custom commands coming together with the printer. So I started searching for software the can spy com ports. There was nothing handy but the Advanced Serial Data Logger. It is even free for a trial period which is ok for me . So I got it started to decode the messages that were being sent from the custom software of the printer. This way I could get more insight on the protocol that was unclear to me and I got the job done.

I want to note just one critical part. I am writing bytes to the port. So I had problems with the cyrillic characters. The page is utf-8 encoded so I had a function that generates the needed bytes to write on the printer like this:

function stringToBytes ( str )
{
  var ch, st, re = [];
  for (var i = 0; i < str.length; i++ )
  {
    ch = str.charCodeAt(i);  // get char
    if(ch >= 1040 && ch <= 1103)
    {
        re = re.concat( [ch - 912] );
    }
    else
    {
        st = [];                 // set up "stack"
        do {
        st.push( ch & 0xFF );  // push byte to stack
        ch = ch >> 8;          // shift value down by 1 byte
    }
    while ( ch );
    // add stack contents to result
    // done because chars have "wrong" endianness
    re = re.concat( st.reverse() );
  }
 }
 // return an array of bytes

 return re;
}

I found this function on stackoverflow but I had to modify it because of the non-latin characters problem. The movement of 912 is when you use utf8. The encoding is different then you have to move the character bits by different ammount.

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