[time-nuts] Lucent KS-24361, HP/Symmetricom Z3809A, Z3810A, Z3811A, Z3812A GPSDO system

Stewart Cobb stewart.cobb at gmail.com
Tue Nov 4 04:38:10 EST 2014

A wiring diagram of the Z3809A cable interconnect cable was published
earlier on this list.  That information appears to be incorrect.  The
cable is actually wired pin 1 to pin 15, pin 2 to pin 14, etc.
Another way to describe it is that for each wire in the cable, the pin
numbers on each end of the cable add up to 16.

A mated pair of these units is running in my lab with a scratch-built
interconnect cable following the above rules.  This scratch-built
cable allowed access to the interconnect signals while the system was
operating happily.  No lights were lit except the green ON light on
the Ref-0 unit (Z3812A, no GPS) and the yellow STBY light on the Ref-1
unit (Z3911A with GPS receiver).  The following signals were observed
on the interconnect (pin numbers given for the J5 interconnect socket
on the Ref-1 unit):

Pin 1:  9600 baud serial data (described below)

Pin 2:  logic low (0.11V)

Pin 3:  Ground (0.00V)  Presence detect? (see below)

Pin 4:  logic high (4.79V)

Pin 5:  inverted Motorola PPS, high (5V) for 800ms, low for 200ms

Pin 6: "17 / 23 dBm" signal from Ref-0 unit (see below)

Pin 7:  logic high (4.48V)

Pin 8:  Ground (0.00V)

Pin 9:  logic low (0.11V)

Pin 10: "17 / 23 dBm" signal from Ref-1 unit (see below)

Pin 11:  inverted PPS, low 400us, high (5V) otherwise

Pin 12:  logic low (0.12V)

Pin 13:  Ground (0.00V)

Pin 14:  logic low (0.08V)

Pin 15:  logic high (4.78V)

Pins 3, 8, and 13 appear to be firmly connected to Ground.  (Note that
these are the three pins which are clipped short on the HP
interconnect cable.)  On an unpowered, disconnected box (either Ref-0
or Ref-1), pins 8 and 13 are connected to Ground (low resistance) and
pin 3 is high impedance.  Presumably pin 3 on each box (connected to
the grounded pin 13 on the other box) is used to sense the presence of
the other box and/or the interconnect cable.

The timing of the PPS signal on pin 11 matches precisely the timing of
the PPS signal available on pins 1 and 6 of J6 (RS422/PPS) on the
active Ref-0 unit.  Presumably this signal is coming across the cable
from the Ref-0 unit.

Note: when the system is coming up from a cold start, SatStat on the
unit with the GPS receiver (Ref-1) will show "[Ext 1PPS valid]" in the
space where it shows "[GPS 1PPS valid]" after the survey is complete.
It appears that the Ref-1 unit timing system is locking its oscillator
to the PPS coming from the Ref-0 unit during this time.

The timing of the PPS signal on pin 5 matches the timing of the PPS
output described in the Motorola OnCore manual.  Presumably this
signal is sourced by the Ref-1 unit to allow the Ref-0 unit to lock to
GPS.  The edges of this PPS signal look very dirty compared to the
signal on pin 11.  This may be an artifact of the homemade cable used
for this experiment.  The HP cable clearly has an overall shield
(visible through the cable sheath) and may have internal coax or
twisted pair for these PPS signals.

When pin 5 and pin 11 are observed together, the usual GPS sawtooth
pattern is evident.

Someone discovered earlier that the both units will blink their green
ON lights if the front-panel switch on either unit is set to 23 dBm
vice the normal 17.  Obviously each unit can communicate its switch
status to the other unit.  They use pins 6 and 10 to do that.  Pin 10
(on the Ref-1 unit) is high (~5V)  if the switch on the Ref-1 unit is
in the 17 dBm position, and low in the 23 dBm position. Pin 6 (on the
Ref-1 unit) gives the same indications for the switch on the Ref-0

The serial data on pin 1 is transmitted at 9600 baud, with a burst of
data every second.  The signal idles at logic low (near 0V) and rises
to logic high (near 5V) during the burst.  This may be the standard
for TTL (not RS-232) transmission of serial data, or it may be
inverted.  The first few characters of one burst were hand-decoded
from a scope trace as 0x40, 0x40, 0x45, 0x61, 0x0B, or ASCII "@@Ea".
This appears to be the Motorola Oncore binary data format, although
"Ea" does not appear to be a valid Motorola command or response.
Perhaps the hand-decoding was in error.

One can use SatStat, talking to the Ref-0 (non-GPS) box, to issue
queries and commands to the GPS receiver.  The results are
inconsistent, but it seems that at least some of the queries get
through and trigger responses.  If the Ref-0 box is actually talking
to the GPS receiver, it must be doing so through the interconnect
cable.  The specific wire in the cable used for this (if any) has not
yet been identified.

An earlier post speculated that the computer in each unit only had two
UARTs.  This does not seem possible.  Clearly each unit uses one UART
to communicate with the J8 diagnostic port.  The Ref-1 unit needs
another UART to communicate with the GPS receiver. And both units need
to be able to transmit the legacy Lucent timecode message out the J6
(RS422/1PPS) port.  Perhaps there is a transmit-only UART coded into
the FPGA, or perhaps one of the UARTs is timeshared with the Lucent
message, or perhaps there is another UART chip hidden somewhere on the

It seems unlikely that the two units are sending serial data to each
other.  (No such data was observed on the interconnect.)  Instead,
they appear to communicate their state to each other by means of logic
levels on various pins of the cable.  The logic functions of pins 6
and 10 have already been identified.  Further research is needed.


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