Parse HEX ASCII into numbers?

Question

I have a H/W device that normally uses a serial port for an interface, sending and receiving binary messages to a PC UI program. I've added an Ethernet port and small TCP/IP stack with a small web server that I want to use to replace the serial port UI with a web browser UI.

The messages are mostly request/response sort of things, but for some web pages I may need to Tx/Rx two or more messages to get all the info I need for the page. I'll use the AJAX XMLHttpRequest() to send the messages and get the responses for a page.

The H/W device has limited resources (CPU & RAM) so to keep it simple on that end I want to just make a little CGI interface that takes outgoing messages and encodes them as HEX ASCII (i.e. two HEX ASCII chars/byte) to send to the browser which will use some java script to pick apart the messages into fields and convert them to numeric vars and display them to the user. Same for messages sent from the browser to the H/W device.

Messages contain a mixture of field types, signed & unsigned bytes, shorts, longs, floats, and are futher complicated by being mostly in little endian byte order in the messages.

I can handle the H/W end code, but I'm struggling to learn java script and could use help with a function to translate the HEX ASCII <-> numerics on the browser end.

Any ideas? Any example code some where?

Thanks, Paul

Answer 1

Sounds like you want parseInt. It takes a string and an optional radix (which should always be supplied), and parses the number as an integer in that radix (or a radix based on the format of the number if none is supplied, which is why you should always supply one; people are surprised to find that parseInt("010") returns 8).

> parseInt("ab", 16)
171

To convert back to hex, you can use toString:

> var num = 16
> num.toString(16)
"10"

Note that you will have to pad it out to two characters yourself if it comes out as only a single character:

> num = 5
> num.toString(16)
"5"

I was bored, so I wrote some functions to do the conversion in both directions for you:

function parseHexString(str) { 
    var result = [];
    // Ignore any trailing single digit; I don't know what your needs
    // are for this case, so you may want to throw an error or convert
    // the lone digit depending on your needs.
    while (str.length >= 2) { 
        result.push(parseInt(str.substring(0, 2), 16));
        str = str.substring(2, str.length);
    }

    return result;
}

function createHexString(arr) {
    var result = "";
    for (i in arr) {
        var str = arr[i].toString(16);
        // Pad to two digits, truncate to last two if too long.  Again,
        // I'm not sure what your needs are for the case, you may want
        // to handle errors in some other way.
        str = str.length == 0 ? "00" :
              str.length == 1 ? "0" + str : 
              str.length == 2 ? str :
              str.substring(str.length-2, str.length);
        result += str;
    }

    return result;
}

Which can be used as follows:

> parseHexString("abcd100001")
[171, 205, 16, 0, 1]
> createHexString([0, 1, 2, 10, 20, 100, 200, 255, 1000, 2000])
"0001020a1464c8ffe8d0"
> parseHexString(createHexString([0, 1, 2, 10, 20, 100, 200, 255, 1000, 2000]))
[0, 1, 2, 10, 20, 100, 200, 255, 232, 208]