(Update to previous  GPS/GlassPanel emails)
(script would typically be saved as GPSReader.tcl)

####################################################################
 # These lines with the '#' character are reader comments.
 # POSSIBLE PROBLEM --- If one tries to include the following script into an email ...
 # Some email programs will insert Line-Feed/Carriage-Retun characters into long lines. 
 # If you want to run the TCL script you might have to first copy the script to a text editor to 
 # remove those characters inserted by your email program.


 # Reads GPS unit connected to USB port and displays data in terminal window.


 set serial [open /dev/ttyUSB0 r]
 fconfigure $serial -mode "4800,n,8,1"
 fconfigure $serial -blocking 0 -buffering none -translation binary




# NMEACode 'GPRMC' identifies the line of NMEA data we want ... "RECOMMENDED MINIMUM SENTENCE"
 
set gpsstring ""


while {1} {                                   ;# False = 0; True = non-zero; therefore we have an infinite-loop  
                                               # TCL allows nesting of commands within [square-brackets]      
set data [read $serial]                       ;# read ALL incoming bytes      
set size [string length $data]                ;# number of received byte (may be 0)     
if { $size } {                                ;# ie ... any size > 0 evaluates as True

append gpsstring $data                        ;# if something received, append data to line of NMEA data

# puts "building: |$gpsstring|"               ;# echo ... debugging - put temp-string on the screen 



# Parse the input data string & locate/index-positons of each data field using substring function
# according to NMEA record layout.

set AsteriskPos [string first * $gpsstring]

if { $AsteriskPos != -1 } {                            ;# the '*' identifies the checksum field near end of line
set NMEADollarPos [string first \$ $gpsstring]
set GPRMCPos [string first GPRMC $gpsstring]           ;# see if line represents RMC data
set GMTPos [expr { $GPRMCPos + 6 }]
set StatusAPos [string first A $gpsstring $GPRMCPos]
set StatusVPos [string first V $gpsstring $GPRMCPos]
set LatNPos [string first N $gpsstring $GPRMCPos]
set LatSPos [string first S $gpsstring $GPRMCPos]
set LongEPos [string first E $gpsstring $GPRMCPos]
set LongWPos [string first W $gpsstring $GPRMCPos]
set SpeedPos [expr { $LongEPos + 2 }]
set TrackPos [expr { $SpeedPos + 6 }]
set DatePos [expr { $SpeedPos + 12 }]
set MagVarPos [expr { $SpeedPos + 19 }]




if { $GPRMCPos > -1 } {                                     ;# Line contains 'GPRMC' and data we want
# puts "final string: |$gpsstring|"                         ;# for debugging
# puts **************
 
set latitude [string range $gpsstring [expr {$LatNPos-10}] [expr {$LatNPos-2}]]
set longitude [string range $gpsstring [expr {$LongEPos-10}] [expr {$LongEPos-2}]]
set speed [string range $gpsstring $SpeedPos [expr {$SpeedPos + 4}]]
set track [string range $gpsstring $TrackPos [expr {$TrackPos + 4}]]


puts "Latitude:  $latitude"
puts "Longitude: $longitude"
puts "Speed: $speed"
puts "Track: $track"
puts **************

         }
       }

set gpsstring [string range $gpsstring $AsteriskPos 200]   ;# re-intialize input string
                                                           ;# save characters read beyond '*' for next line
     

update;# Display output, allow to close wish-window
  }  ;# end found * near end of line.
 }  ;# end size loop ... ie real non-null data read from USB port
}  ;# end while infinite loop