Rob's Trimble GPS Project

In ~1997, I acquired two Trimble GPS receivers. As a navigation systems consultant, by profession, I welcomed the opportunity to 'play' with them.

Outside view - click on thumbnails for full size picture


Inside view - click on thumbnails for full size picture

Hardware Details

As you can see from the outside pictures, the unit on the right (part number: 22442-02) is labelled as a Placer 400. According to the Placer 400 Installation & Operator's manual (which refers to part number: 22442), it is capable of receiving DGPS corrections. Unfortunately the unit on the left (part number: 21589-00) has no such label - so is more of a mystery. An E-mail from Trimble indicated that it was equivalent to the SVeeSix. As you can see from the inside pictures, neither unit has a daughterboard - so don't match with the diagrams in the SVeeSix-CM3 Embedded GPS Core Module System Designer Reference Manual. The SVeeSix-CM3 Embedded GPS Core Module System Designer Reference also specifies a part number of: 34055 - so the unit on the left clearly isn't one of these. However, internet searching has indicated that the Placer 400 is basically just an SVeeSix built into a motherboard with standard voltage communication ports, and the 21589 unit is effectively an early (c1990) version of this - being an early SVeeSix starter kit.

Both units initially had the same 3-pin plastic male power connector, and the corresponding female connector does not latch very securely onto the male. Consequently, when preparing to use the 21589 in-car, I replaced its connector with a 4-pin metal military connector, which latches very securely indeed. This can be seen in the left hand picture.

Both units also have 2 female D9 connectors, for communication using the RS232 byte level hardware protocol. The pins on these connectors are wired such that a 'straight through' cable will work directly with a standard PC serial port.

As you can see from the pictures, the 21589 is fitted with an SMA antenna cable, whereas the Placer 400 is fitted with an SMB antenna cable. SMA is much more common in other applications. The 21589 had a blanked hole next to the SMA connector, which I filled with a jumbo LED - wired to the main power supply lines.

Both receivers appear to have 6 tracking channels and only track C/A code (SPS) data on L1.

For in-car use, I raided my collection of old mobile phone cigarette lighter power cables, and replaced the mobile phone connector with the military connector that mates with the one I fitted to the 21589. This has an LED of its own and works beautifully in its new application.

Software Configuration

I initially used a version of TSIPChat to communicate with the receivers, as they both had a communications port that implements the Trimble Standard Interface Protocol (TSIP) - a binary message level protocol. However, I have since written my own software to issue TSIP commands and display data from the receivers. Interestingly, TSIPChat fails to instruct the 21589 to do anything, but my software successfully commands it to do everything I've so far implemented! I haven't got to the bottom of this yet...

As an aside, I had first thought that the units communicated via the TIPY message protocol. The TIPY protocol appears identical to TSIP except that TIPY includes a message length byte, whereas TSIP does not. I wonder why Trimble developed two such similar protocols...

The 21589 is loaded with an earlier software version than the Placer 400, and broadcasts NMEA messages GGA and VTG by default from start up on Port 1. After loss of power, the Placer 400 needs to be commanded to output NMEA messages on Port 1, via an appropriate TSIP command issued via Port 2.

By running programs downloaded from the Trimble FTP site, I have managed to change the message type used by the Placer 400 Port 2 from TSIP to NMEA, and back. These changes withstand complete loss of power. However, I have never managed to get either receiver to output other than just the GGA and VTG messages. The TSIP reference document indicates that others are possible, but neither receiver responds to the appropriate TSIP command to turn then on. Nor does the command to turn off GGA and VTG work either! The SVeeSix-CM3 Embedded GPS Core Module System Designer Reference Manual says "Although the Trimble CM3 supports seven NMEA sentences that contain GPS information, the standard CM3 only outputs the GGA and VTG data strings. (Note: Contact your Trimble sales representative if you need access to all or a subset of the other five NMEA sentences)." I asked Trimble about this but they don't support these devices any more and said they couldn't help. One E-mail that I received from Trimble said that the Placer 400 only spoke TAIP (not TSIP or NMEA) - which is clearly rubbish!

Application

For the in-car holiday project (which pre-dated some of my hardware refinements), I installed OziExplorer on my PDA, and made an adapter so that the standard PDA<->PC RS232 cable would work with the wiring of the receiver ports. Once OziExplorer was set to use GGA NMEA messages at 4800 baud, it happily showed our position overlaid on digital maps that I had collaged from internet sources and georeferenced using the PC based OziExplorer software. OziExplorer also recorded our position over time and provided fairly conclusive proof that parts of the Michelin map of France were suffering from a registration error!

Almanac

TSIPChat has the option to download, from the receiver to the PC, an almanac for all 32 GPS satellites. The almanac is stored on the PC in a binary file called 'GPSALM.DAT'. This is a non-standard format and not described anywhere that I could find. Trimble also failed to respond to my requests for software to convert it to a standard format (e.g. SEM or YUMA), so I worked out the format and wrote my own program to read the almanac data from the binary file and create an equivalent YUMA format almanac file. The resulting files successfully import into the Trimble Planning software, and show the correct satellite positions.

Abbreviatons

C/A  GPS civilian Coarse Acquisition code
GPS  Global Positioning System
IBM  International Business Machines
LED  Light Emitting Diode
NMEA  National Maritime Electronics Association
PC  (IBM-compatible) Personal Computer
PDA  Personal Digital Assistant (Pocket PC)
SPS  Standard Positioning Service
TAIP  Trimble ASCII Interface Protocol
TIPY  Trimble Interface Protocol ?
TSIP  Trimble Standard Interface Protocol

References

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