Notes on Hooking Up to Other Pieces of Equipment to Your Guidance System GPS
By Randy Price
K-State Extension and Research
Introduction: It is highly likely that your GPS
Figure 1: Typical 9-Pin RS-232
signal will be derived from a guidance system or
Connector
a stand alone GPS unit. In either case, you may
want to pipe (or send) this information to other
pieces of equipment (such as an Insight monitor
or PDA, etc.). Typically this is done through a
9-Pin RS-232 port on the back of the unit (USB
cabled GPS units are available, but not as
common since they must have a driver loaded to
communicate correctly with the piece of
equipment, making them not as practical for plug and go type operations). Two make an
RS-232 connection, two operations must occur: Getting the right cable (null modem or
straight through) and setting up the equipment to transmit at the same baud rate and
sentence structure.
Cabling: Pin outs for RS-232 connectors are shown in Table 1. These connectors come
in two styles: DTE and DCE depending upon whether the equipment was set-up to
transmit signals or receive signals. Cabling for the RS-2323 connector exists in two main
types: a null-modem cable and a straight through pass cable. Typical in most cases, only
three pins are needed (pins 2 and 3 - transmit and receive - and pin 5 - ground) to make a
working connection (note though that in some equipment, DTR and DSR lines may be
needed). Null-modem cables switch the transmit and receive pins (numbers 2 and 3)
while a straight through cables do not. Depending upon the cable you have, you may
have to use a null-modem adapter which switches pins 2 and 3 for you, and a gender
changer, which switches the type of connectors (male/female) to fit (I frequently have to
use both in series to make a connection). These connectors are available locally at Radio
Shack (although not that cheap - $7 or more). If you are unsure which cable you need,
carry a straight through cable and have a null modem adapter and gender changer ready
to flip pins 2 and 3 (switching pins 2 and 3 will not hurt any equipment since these are
only digital lines). Note that if your equipment supplies power through other pins on the
cable, you may have “break” pins off to not transmit this power (possibly causing
grounding or shorting). I once had an Insight monitor that sent power and ground across
several of the pins on the 9-pin cable, and had to make an adapter using this technique to
only connect pins 2, 3, and 5. In some cases (but not very often) you may need leave pins
6-8 connected for other communications (DTR, DSR, etc.), but most RS-232 connections
only need pins 2,3, and 5 for transmitting and receiving. Screwing in connectors will help
prevent problems in connection as many connectors may pop-out and lose connection,
while still looking connected. Worn connectors may suffer the same problem.
Baud Rate and Sentence Structure: Once a cable has been formed, the second step is to
set up the equipment to operate at the same baud rate and sentence structure. Some
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receiving pieces of equipment do this automatically, but it is not very common, and
typically you have to set these to match. This process involves specify the baud rate
(typically 4800 to 115,000), bits per word (7 or 8), and parity (even or odd). These is
typically done somewhere in menu, software, or switches. The most common sentence
structures is 4800, 8, N, 1, but some newer GPS will use 9600, 19200, or even 38400
baud, and some older equipment may use 7 bits transmission and no parity transmission.
Check the manual to make find out. You can try different baud rate, etc., but this usually
takes to long unless you have a good idea what it should be. Note that if cables are
extremely long, or have bad connections, you may have to reduce the baud rate for the
cable to transmit data (this is usually not recommended since bouncing of the vehicle,
etc., will typically cause transmitting problems).
Other Ways to Connect: Most equipment should work with the techniques above. If you
still have problems, the circuit below may help if the equipment is need the extra lines to
complete handshaking.
Source: AirBorn Electronics
Table 1: Typical Pin Specifications for the 9-Pin RS-232 Plug
RS-232 9-pin signal definition for the DTE device - looking
into the DTE connector*
DTE RS232 device is often a PC.
Pin No
Circuit name
Abbreviation
1
Received line signal detect
2
Receive data
RXD
3
Transmit data
TXD
4
Data terminal ready
DTR
5
Signal ground
2
Source
6
Data set ready
DSR
7
Request to send
RTS
8
Clear to send
CTS
9
Ring indicator
DCE
RS-232 9-pin signal definition for the DCE device - looking
into the DCE connector*
DCE device is often a modem.
Pin No
Circuit name
Abbreviation
1
Received line signal detect
2
Transmit data
TXD
3
Receive data
RXD
4
Data terminal ready
DTR
5
Signal ground
6
Data set ready
DSR
7
Clear to send
CTS
8
Request to send
RTS
9
Ring indicator
Source
DCE
Source: Radio-Electronics.com
*Notes:
- The most important pins are shown in green.
- Only the transit and receive pins are switched between DTE and DCE connectors.
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