• PC Enginges board, with DCD line (APU = ok, ALIX = not ok)
• Debian 10 installed.
• Getty on ttyS0 disabled.
• APU1 to Hopf 7001 system connected over serial (see wiring below).
• Hopf 7245 interface card configured for NTP (see 7245 interface board documentation).
• NTP source code download.

• Hopf documentation
• Hopf 7001DCF77 system
• Hopf 7245 interface board
• PC Engines APU1 board

• Interface board 7245 DIP-switch settings.

  1 = on 
  0 = off
  
  SW1      SW2      SW3
  12345678 12345678 12345678 
  11111001 11000111 11111110

• Construct a serial cable to route PPS to DCD.

  Hopf 7245     1 GND --------- 5 GND     PC Engines
  S1 RS232      2 TXD --------- 2 RXD     APU1 board
                6 RTS (PPS) --- 1 DCD
  

• Install required tooling.

  apt install build-essential libcap-dev pps-tools

• Create new udev rules file.

  cat > /etc/udev/rules.d/hopf.rules <<EOF
  KERNEL=="ttyS0", SYMLINK+="refclock-0"
  KERNEL=="ttyS0", RUN+="stty -F /dev/%k 9600 cs8 -cstopb -parenb"
  KERNEL=="ttyS0", TAG+="systemd", ENV{SYSTEMD_WANTS}="pps.service"
  KERNEL=="pps0",  SYMLINK+="refclockpps-0"
  EOF

• Reload the udev rules or reboot to test.

  ls -l /dev/refc*
  lrwxrwxrwx 1 root root 5 Jan  4 14:37 /dev/refclock-0 -> ttyS0
  lrwxrwxrwx 1 root root 4 Jan  4 14:37 /dev/refclockpps-0 -> pps0
  
  ppstest /dev/pps0
  trying PPS source "/dev/pps0"
  found PPS source "/dev/pps0"
  ok, found 1 source(s), now start fetching data...
  source 0 - assert 1578145727.901770116, sequence: 1313 - clear  1578145072.692767609, sequence: 2
  source 0 - assert 1578145728.000351551, sequence: 1314 - clear  1578145072.692767609, sequence: 2
  source 0 - assert 1578145728.901731373, sequence: 1315 - clear  1578145072.692767609, sequence: 2
  source 0 - assert 1578145729.000557999, sequence: 1316 - clear  1578145072.692767609, sequence: 2
  ^C
  

• Disable and remove other time sync applications.

  apt-get remove --purge ntp chrony
  timedatectl set-ntp false
  systemctl stop systemd-timesyncd
  systemctl disable systemd-timesyncd

• Add ntp user.

  groupadd ntp
  useradd -c "Network Time Protocol" -d /var/lib/ntp -g ntp -s /bin/false ntp
  usermod -a -G dialout ntp
    * Create directories.<code>
  install -v -o ntp -g ntp -d /var/lib/ntp
  install -v -o ntp -g ntp -d /var/log/ntpstats

• Compile source code.

  tar zxf ntp-4.2.8p13.tar.gz 
  cd ntp-4.2.8p13
  
  # from Linux From Scratch
  ./configure CFLAGS="-O2 -g -fPIC" \
              --prefix=/usr         \
              --bindir=/usr/sbin    \
              --sysconfdir=/etc     \
              --enable-linuxcaps    \
              --enable-parse-clocks \
              --with-lineeditlibs=readline \
              --docdir=/usr/share/doc/ntp-4.2.8p13
  
  make
  
  make install

• Add systemd services.

  cat > /etc/systemd/system/ntpd.service <<EOF
  [Unit]
  Description=Network Time Service
  After=syslog.target ntpdate.service sntp.service
  Conflicts=systemd-timesyncd.service
  
  [Service]
  Type=forking
  ExecStart=/usr/local/bin/ntpd -g -u ntp:ntp
  PrivateTmp=true
  
  [Install]
  WantedBy=multi-user.target
  EOF
  
  systemctl enable ntpd
  systemctl start ntpd
  
  
  cat > /etc/systemd/system/pps.server <<EOF
  [Service]
  Type=simple
  ExecStart=/usr/bin/ppsldisc /dev/ttyS0
  EOF

• Create /etc/ntpd.conf.

  #Stratum level when no ref source available
  tos orphan 12
  
  # Hopf DCF clock
  # Mode 12 + 128 (pps) = 140
  server 127.127.8.0     mode 140
  
  fudge  127.127.8.0     refid PPS       # Reference identifier
  
  #leapfile /etc/leap_second
  
  restrict -4 default notrap nomodify nopeer noquery
  restrict -6 default notrap nomodify nopeer noquery
  restrict 127.0.0.1
  restrict ::1
  
  statsdir /var/log/ntpstats/
  statistics loopstats peerstats clockstats
  filegen loopstats file loopstats type day enable
  filegen peerstats file peerstats type day enable
  filegen clockstats file clockstats type day enable
  
  driftfile /var/lib/ntp/ntp.drift
  

ntpq -c 'cv 0 timecode'
timecode="\x02CB154556010120\x0a\x0d\x03"

#!/bin/bash
#
# Tested with Hopf 7001 DCF Base station with 7245 serial interface boards
# Datastring 7001/6021
#
# BS, 2013/05

ntpq="/usr/bin/ntpq"

# get timecode variable, received from the clock
eval `$ntpq -c 'cv 0 timecode'`

function quit {
	case "$2" in
	0)	echo "OK: $1"
		exit 0
		;;
	1)	echo "Warning: $1"
		exit 1
		;;
	2)	echo "Critical: $1"
		exit 2
		;;
	3)	echo "Unknown: $1"
		exit 3
		;;
	esac
}

function clockstat {
	# Status nibble from hex
	hex0=0000
	hex1=0001
	hex2=0010
	hex3=0011
	hex4=0100
	hex5=0101
	hex6=0110
	hex7=0111
	hex8=1000
	hex9=1001
	hexA=1010
	hexB=1011
	hexC=1100
	hexD=1101
	hexE=1110
	hexF=1111

	eval statnibble='hex$1'
	snibble=${!statnibble}

	case ${snibble:0:2} in
	00)
		textstatus="time/date invalid"
		exitcode=2
		;;
	01)
		textstatus="crystal operation"
		exitcode=1
		;;
        10)
                textstatus="radio operation"
		exitcode=0
		;;
	11)
		textstatus="radio operation (high accuracy)"
		exitcode=0
		;;
	esac

	case ${snibble:2:1} in
	0)
		dss="standard time"
		;;
	1)
		dss="daylight saving time"
		;;
	esac

        case ${snibble:3:1} in
        0)
                ann="no announcement hour"
                ;;
        1)
                ann="announcement (ds-st-ds)"
                ;;
        esac

	quit "status hex=\"$1\", bin=\"$snibble\": $textstatus, $dss, $ann" $exitcode
}


# check for complete string, for example: \x028D194223310513\x0a\x0d\x03
# This is from the Hopf 7245 serial interface board
#
# x20 = Space
# x0D = CR (carriage return)
# x0A = LF (line feed)
# x02 = STX (start of text)
# x03 = ETX (end of text)
#
if [[ $timecode == \\x02*\\x0a\\x0d\\x03 ]];
then
	clockstat ${timecode:4:1}
else
	quit "Timecode not complete" 3	# return Unkown status
fi

• Show peer status:

  ntpq -p

• Show timecode from receiver:

  ntpq -c 'cv 0 timecode'

• Show client list (max. 600):

  ntpdc -n -c monlist
  
  ## ntp 2.4.7 removed monlist in favour of ntpq's mrulist
  ntpq -c "mrulist"