usbscale.pl

#!/usr/bin/perl
#
# Eric Jiang
# http://notes.ericjiang.com/posts/54
# This software is public domain.
#
# NOTE: This code is not maintained!
# There is a newer version written in C:
# https://github.com/erjiang/usbscale
#
use bytes;

my $data;

#prevents us from repeating messages
my $waitingflag = 0;

while (1) {

    $data = `cat /dev/hidraw4 | head -c 7`;

    my $report = ord(substr($data, 0, 1));
    my $status = ord(substr($data, 1, 1));
    my $unit   = ord(substr($data, 2, 1));
    my $exp    = ord(substr($data, 3, 1));
    my $lsb    = ord(substr($data, 4, 1));
    my $msb    = ord(substr($data, 5, 1));
    my $weight = ($msb * 256 + $lsb) / 10;
    if($exp != 255 && $exp != 0) {
        $weight ^= $exp;
    }
    #print "$report $status $unit $exp $weight\n";

    if($report != 0x03) {
      die "Error reading scale data!\n";
    }

    if($status == 0x01) {
      die "Scale reports FAULT!\n";
    } elsif ($status == 0x02 || $weight == 0) {
        if($waitingflag != 0x02) {
            print "Zero'd...\n";
            $waitingflag = 0x02;
        }
    } elsif ($status == 0x03) {
        if($waitingflag != 0x03) {
            print "Weighing...\n";
            $waitingflag = 0x03;
        }
    } elsif ($status == 0x04) {
        my $unitName = "units";
        if($unit == 11) {
            $unitName = "ounces";
        } elsif ($unit == 12) {
            $unitName = "pounds";
        }
        print "$weight $unitName\n";
        last;
    } elsif ($status == 0x05) {
        if($waitingflag != 0x05) {
            print "Scale reports Under Zero...\n";
            $waitingflag = 0x05;
        }
    } elsif ($status == 0x06) {
        if($waitingflag != 0x06) {
            print "Scale reports Over Weight!\n";
            $waitingflag = 0x06;
        }
    } elsif ($status == 0x07) {
        if($waitingflag != 0x07) {
            print "Scale reports Calibration Needed!\n";
            $waitingflag = 0x07;
        }
    } elsif ($status == 0x08) {
        if($waitingflag != 0x08) {
            print "Scale reports Re-zeroing Needed!\n";
            $waitingflag = 0x08;
        }
    } else {
        die "Unknown status code: $status\n";
    }

}

FYI, I just upgraded to Ubuntu 10.10 and the script stopped working. A bit of poking and it seems that you can make it work by shifting the variable indices down by 1:

my $report = ord(substr($data, 0, 1));
my $status = ord(substr($data, 1, 1));
my $unit   = ord(substr($data, 2, 1));
my $exp    = ord(substr($data, 3, 1));
my $lsb    = ord(substr($data, 4, 1));
my $msb    = ord(substr($data, 5, 1));

@mattismyname My first reaction is that it doesn't make sense for the variable indices to be shifted in 10.10 unless the USB HID protocol changed or Perl suddenly switched to 1-based indices or something weird. I haven't tried 10.10 yet but I will double-check this script when I do.

Yes, it is weird. Maybe the hidraw kernel driver changed...?

I can confirm the problem + solution for Fedora 13, 64-bit (not sure if that matters). Shifting the indices down by one made it report the weights again.

The indices for the scale data have been shifted down, per mattismyname's investigation.

Thanks! I'll give this a try and get back with you.

I have just finished an early version of a C program that reads USB scales. Try it out and let me know how it goes. https://github.com/erjiang/usbscale

Consider using unpack() instead of ord(substr()). Example here.

Hello, I am trying to use this script with a Metler Toledo PS60. I am able to get the script to talk with the scale, but it is reading the weight incorrectly. For example, a 4lb item will read as 255lb, how in the script can I possibly correct this?

thanks

Just a note, this can be adapted for Fairbanks scales by removing the line $weight ^= $exp; and changing / 10 to /100

@bhartvigsen Interesting find. Can you open an issue on https://github.com/erjiang/usbscale/issues with details about your scale? It'd be nice to add support for it.