[time-nuts] Ball/Efratom MFS-209 Rubidium GPSDO...

[Chuck Harris]

Hi gang!

I am in the process of repairing a Ball/Efratom MFS209 GPSDO.

The MGPS unit reports that I have an ADC fault, and a feed line
fault.

I have done some testing and can easily see that the feed line has
the proper +5V on the unit's N connector, and that it can source
the required 200ma, but with a known good antenna, the MGPS
controller still reports the fault.

The MGPS communicates with an internal Trimble GPS receiver board
using RS422, and the MGPS pins the "-" input/outputs of the RS422
signal to ground, the communication is essentially TTL RS232.

The ADC, and the detector for the feed line faults are on the
Trimble board, so I thought I would sniff at the serial lines to
see what packets were being sent to and from the GPS.  I tagged a
couple of wires onto the interface connector for the Trimble board,
and hooked them up to a scope, and verified that they were 0-4V
and 9600 baud, and then hooked up a protocol analyzer to capture some
messages.

Since the Ball/Efratom advertising blerbs I found on John Ackermann's
website say it is a 6 channel GPS receiver, and at that time, Trimble
made a common 6 channel receiver called the SVeeSix, I went there for
hints on the communication protocol.  Trimble provides a serial 8 bit
protocol called TSIP on their GPS engines, so I looked at that first.

TSIP is a packet protocol, which uses frames that look like this:

<DLE><packet ID><packet data bytes...><DLE><ETX>

Where DLE is an ascii 0x10, the packet id is one or two bytes and ETX
is an ascii 0x03.

I get the following packet, sent by the Trimble, about every 15 secs:

FF 90 14 20 20 20 06 41 96 65 1E 01 41 06 FF

After 3 such packets, the MGPS sends:

FF 90 22 D0

Then immediately the Trimble sends:

FF D0 24 01 01 01 01....01 01 41 06

Where there are 25 of the "01" bytes in a row.

and the MGPS again sends the packet:

FF 90 22 D0

After which the whole process repeats.

Clearly, this isn't regulation TSIP!

The other protocols proffered by Trimble send plain text messages
intended for printing, so it isn't one of them (TSIA, or NMEA).

Does anyone recognize this protocol?  I have a little trouble believing
that Ball/Efratom got Trimble to roll a custom data protocol just
for this MGPS unit, but documentation is so sparse, that who knows?

Any ideas?

-Chuck Harris