Port2 of this GPS clock is specific to just this model and gives a 16 byte pulse once every other second with Time-of-Day in a binary format.
The data is shown in Hex:
00 09 01 07 03 01 04 04 00 02 03 01 03 00 00 0D
All numerical data is transmitted as a single byte per digit, i.e. digit 9 is transmitted as 09 Hex.
Bytes 1-2 are two least significant digits of the Year (00 09) 2009.
Bytes 3-5 are the numerical Day of the Year (01 07 03) or day 173 (June 22). Zero starts as Jan 1
Bytes 6-7 are the Hour (01 04), hour 14 or 2PM.
Bytes 8-9 are the Minute (04 00), minute 40.
Bytes 10-11 are the Second (02 03), second 23.
Bytes 12-13 are the Accumulated Leap Seconds (01 03), 13 leap seconds.
Bytes 14-15 SmartClock Mode or Status: 01 00: Power-Up Mode, 10 00: Holdover Mode, 00 00: GPS Lock Mode
Byte 16 is a Carriage Return (0D).
Under the example above, the Date and Time is June 22, 2009 (day 173) at 14:40:23, and no Holdover. On the Port2 serial interface, the serial port parameters are fixed at 9600, N, 8, 1 and cannot be changed. Although the Port2 connector is wired for Receive Data (pin 3) it does not appear that the GPS Receiver will accept any commands. All SCPI commands must be issued over the Port1 serial interface connector.
When the last byte (0D) is received, sync should happen then which lines-up with Tcl's trigger for a readable event quite well.
The test script prints this every 2 seconds
Wed Sep 30 07:51:33 PDT 2015: GPS Locked Wed Sep 30 07:51:35 PDT 2015: GPS Locked Wed Sep 30 07:51:37 PDT 2015: GPS Locked Wed Sep 30 07:51:39 PDT 2015: GPS Locked Wed Sep 30 07:51:41 PDT 2015: GPS Locked Wed Sep 30 07:51:43 PDT 2015: GPS Locked Wed Sep 30 07:51:45 PDT 2015: GPS Locked Wed Sep 30 07:51:47 PDT 2015: GPS Locked
set serial [open /dev/ttyUSB0 r+]
fconfigure $serial -mode "9600,n,8,1" -handshake none -blocking 0 \
-buffering line -translation cr
fileevent $serial readable [list serial_receiver $serial]
proc serial_receiver { chan } {
set data [gets $chan]
if {![string length $data]} {
if {[eof $chan]} {
# get everything left
set data [read $chan]
puts stderr "Closing $chan"
catch {close $chan}
} else {
return
}
}
puts [translate $data]
}
proc translate {bytes} {
# decode the 15 byte packet
binary scan $bytes {c2c3c2c2c2c2c2} year doy hr min sec lsec mode
# collapse the lists into integers
foreach var {year doy hr min sec lsec mode} {
set $var [expr {[join [set $var] ""]}]
}
# day-of-year starts at zero, but Tcl wants one
set doy [expr {$doy + 1}]
# convert to UNIX time seconds. Make sure the radio is set
# not to convert timezone using :PTIME:TZONE 0,0 on port1
set seconds [clock scan \
"$year $doy $hr $min $sec" -format {%y %j %H %M %S} -gmt 1]
# subtract leap seconds
set seconds [expr {$seconds - $lsec}]
switch -- $mode {
0 {set state "GPS Locked"}
10 {set state "Power-Up"}
100 {set state "Holdover"}
}
return "[clock format $seconds]: $state"
}