Hi Bruce,
Progress report 2:
The half duplex handshaking turned out to be a red herring.
It is now working  <grin>
The prefix of "\x03" needed to be added to the
outgoing string in "lib/Serial_Item.pm".  Adding it to the front of the text in "items.mht" doesn't work.
I added the following lines (filename in brackets)
         my $datatype  = $main::Serial_Ports{$port_name}{datatype};
        my $prefix  = $main::Serial_Ports{$port_name}{prefix};
        $serial_data .= "\r" unless $datatype and $datatype eq 'raw';
        $serial_data = $prefix . $serial_data if $prefix ne '';
        my $results = $main::Serial_Ports{$port_name}{object}->write($serial_data);
  Added an extra parameter (_prefix) to the
serial_port_create function.
.. but that didn't work either...
In order for it to work, I've had to revert to
(lib/Serial_Item.pm) adding the prefix manually..
$serial_data = "\x03" . $serial_data;
I'm guessing that something is stopping the prefix
from being passed around as a $3 byte.

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