Added support for "Conrad DCF-77 mobil" radio clock. This is the cheapest

(DM130) way to get precise ticks if you are within range of the xmitter in
Mainflingen.  I'm some 1000Km away and have +/- 2ms clock using this.

usr.sbin/xntpd/README.FreeBSD

@@ -8,3 +8,71 @@ in order to make this code exportable.  If you have a legal copy of
 to the AUTHDEFS in Makefile.inc.
 
 You can change CLOCKDEFS in the same file to add other reference clocks.
+
+----------------------------------------------------
+Support for Conrad electronic's "DCF-77 Uhr, Mobil".
+----------------------------------------------------
+Conrad electronic in Germany,, Phone (+49) 962230111 (?), sells a gadget
+called "DCF77 Uhr, mobil", which is a DCF77 timecode receiver with a
+rs-232 interface.  The price is around DM130.
+	9-pin interface is Order# 97 94 57 66
+	25-pin interface is Order# 97 94 81 66
+
+You must define
+	-DDCF77 -DPPS -DFREEBSD_CONRAD -DDEBUG
+when you compile xntpd.  You can later remove -DDEBUG, if you feel like it.
+
+You must also have
+	options         COM_BIDIR
+defined in your kernel, and finally the ttyport you intend to use must
+have special interrupt vector:
+	device sio1 at isa? port "IO_COM2" tty irq 3 vector siointrts
+							 ^^^^^^^^^^^^
+connect the radio-clock to the tty port and link it to /dev/refclock-0:
+
+	cd /dev
+	sh MAKEDEV cua1
+	ln -s /dev/cua01 /dev/refclock-0
+
+make a directory to gather statistics in:
+	mkdir /var/tmp/ntp
+
+Create a /etc/ntp.conf along these lines:
+
+	# DCF77 without PPS
+	server 127.127.8.20
+	# DCF77 with PPS
+	#server 127.127.8.148 prefer
+
+	driftfile /var/tmp/ntp/ntp.drift
+	statsdir /var/tmp/ntp
+	statistics loopstats
+	statistics peerstats
+	statistics clockstats
+	filegen peerstats file peerstats type day enable
+	filegen loopstats file loopstats type day enable
+	filegen clockstats file clockstats type day enable
+
+Try to start it:
+	comcontrol ttyd1 bidir
+	tickadj -A
+	xntpd -d -d -d
+
+You should see the red LED flash on the receiver every second now.  You
+may have to experiment a bit with the location, and possibly adjust the
+minute variable resistor inside to get a good signal.  Be aware, that just
+because you see the light flash, is not the same as the signal being
+received by the computer.  The chip doing the work in the reciver uses
+less than 1 micro-ampere, so even if RTS isn't pulled low, it will happily
+receive, but be unable to buffer the signal to the rs-232 levels needed.
+
+You can see what's going on in /var/log/messages, and query the
+daemon using xntpdc and ntpq, in particular the "clockvar" command
+of ntpq will tell about the clocks healt.
+
+I live in Slagelse, Denmark, which is ~1000 Km from Mainflingen, yet
+I have +/- 2 ms precision from this cheap gadget.  If you have a very
+stable signal, you can use the 'pps' address instead to improve your
+timing.
+
+Have fun...  Poul-Henning Kamp <phk@login.dkuug.dk>