I have an older serial based LCD display (2x16char) and a mini-ssc (serial servo controller by Scott Edward) connected together. To move the servos I need to send the following 5 bytes: 

\xFE\x90     #This moves the LCD 'cursor' off screen
\xFF\x00\x05 #This tells the SSC to move the first servo to
             #position 5 (range of 0-254 acceptable, but I
             #will limit that to 253 to avoid the LCD 
             #display).

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I am using the Device::SerialPort module from CPAN and am confused by what appears to be magic type casting.

Here's my test code. The first section works, the loop does not. Why? 

#!/usr/bin/perl
use Device::SerialPort 0.12;

$ob = Device::SerialPort->new ("/dev/ttyS1") || die "S1 dead";
$ob->datatype("raw") || die "raw";
$ob->baudrate(2400) || die "baudrate";
$ob->parity("none") || die "parity";
$ob->databits(8) || die "databits";
$ob->handshake("none") || die "handshake";
$ob->write_settings || die "write settings";

#write to both lines of the LCD display
#clear screen
$ob->write("\xFE\x01");
$ob->write("Hello");
$ob->write_drain;

#second line
$ob->write("\xFE\xC0");
$ob->write("World");
$ob->write_drain;

# These lines move the servo to 5 different positions
sleep(1);
$ob->write("\xFE\x90\xFF\x00\x01");
$ob->write_drain;

sleep(1);
$ob->write("\xFE\x90\xFF\x00\x11");
$ob->write_drain;

sleep(1);
$ob->write("\xFE\x90\xFF\x00\x21");
$ob->write_drain;

sleep(1);
$ob->write("\xFE\x90\xFF\x00\x31");
$ob->write_drain;

sleep(1);
$ob->write("\xFE\x90\xFF\x00\x41");
$ob->write_drain;

#This loop should move the servo 
#nearly full range in 25 step
#it doesn't.

$x = 25;
while ($x>0) {
 sleep(1);
 $ob->write("\xFE\x90\xFF\x00");
 $ob->write(int($x*10));
 $ob->write_drain;
 $x--;
}
#end

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I have tried a couple of variations for sending the value. 

$fs = sprintf "\\xFE\\x90\\xFF\\x00\\x%X", int($x*10);
$ob->write($fs); # does not appear to work

[download]

In reply to Casting magic - how to write numbers to the serial port by ehud42

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