How to use gpgdragon rx tx cable
Note:GarminVario - the ultimate Garmin GPS cable and sources of supply for special cables and connectors
A generic approach:
GPS receivers with a serial interface and NMEA protocol (or another such as Garmin or Rockwell protocol) are relatively easy to connect to computers, as every computer has a serial interface. The entire procedure is described below.
To avoid misunderstandings, the interface signals are clearly marked:
Tx = transmit / send, Rx = receive / receive, often also with TxD = TransmitData and RxD = ReceiveData, or simply called TD or RD.
Note: For a functioning communication, Rx must always be connected to Tx or vice versa. TX-TX or Rx-Rx CANNOT WORK!
Tx (gps) is the serial output of the GPS receiver. Here the receiver outputs the serial coordinate protocol. This is the actual signal that we need to couple the GPS receiver and the computer, it is absolutely necessary and with Rx (comp) connected to the computer.
Rx (gps) is the serial input of the GPS receiver. In many cases it is not used. Possible uses are, for example, the transmission of waypoints from a computer IN to the GPS receiver, or the configuration of the receiver via so-called NMEA input msgs. This can be used, for example, to configure the date used, the type of NMEA msgs output and, if necessary, a receiver initialization. Usually not required for establishing a connection between the GPS receiver and the computer. If used, it is connected to Tx (comp)
Tx (comp) is the send output of the computer. If used, it is connected to Rx (gps) to transmit waypoints or to configure the receiver. Usually not required for establishing a connection between the GPS receiver and the computer.
Rx (comp) is the serial input of the computer. Here the computer receives the serial data sent by the GPS receiver on Tx (gps). This connection is absolutely necessary.
GND / Logical GND is the electrical reference point for the communication signals Rx and Tx. Every RS232 interface requires such a reference point, because without it the RS232 level + -12V are not defined. When connecting two RS232 interfaces, the GND pins on each side are simply connected to one another.
Usually there is a ground or shield connection in addition to the GND / Logical GND pin. This is often connected to GND / LogicalGround and can be used as such. There are exceptions, however, so in case of doubt you should always use the Logical GND connection. Many simple RS232 interfaces, e.g. the connection on the Garmin GPS12, do not have separate logical GND and ground connections. In this case you have no choice anyway.
Logical GND is on pin 5 of the DSUB-9 connector, and on pin 7 of the DSUB25 connector. Ground / shield, on the other hand, is on the connector / socket housing and is connected to the metal housing of the computer in conventional PCs.
Level 1: The interface level
Most GPS receivers have a standard RS232 interface with the usual + -12V levels (this can also be a little less or more, e.g. + -6V). Many OEM boards and some GPS sensors have a simple CMOS / TTL interface with only 0V / 5V or even 0V / 3.3V voltage swing. With such receivers you should use a downstream level converter, e.g. from MAXIM, to establish standard RS232 level ratios. Some GPS OEM boards, such as the GPS25 from Garmin, are available in different versions, the GPS25HVS and GPS25LVS, for example, have standard RS232 levels, the GPS25HVC and GPS25LVC have CMOS / TTL levels. As a rule, you should choose the version with RS232 level, as this is the easiest way to connect to the computer. An exception is the connection of a GPS receiver with CMOS / TTL level directly to a uC / processor, which itself also works with CMOS / TTL level. In this case the RS232 level requires an unnecessary level conversion.
RS232 levels are intended for longer cable connections (0.5-50m), while CMOS / TTL levels are only intended to bridge short direct connections in the range of a few cm.
With some computers, the serial interfaces are special cases: Apple computers, for example, do not have a real RS232 interface, but so-called RS422 or RS423 interfaces. The interface signals are named differently than with RS232, and at first glance a connection between GPS RS232 and computer RS422 / 423 seems impossible. In fact, the interfaces are very largely compatible, and a connection to the GPS receiver is always possible, even without additional level converters.
Level 2: The cable assignment
The usual computers have RS232 interfaces, which in addition to the actual data lines (RxD-ReceiveData / TxD-TransmitData) also use additional so-called handshake lines, e.g. RTS / CTS, DTR, etc. These handshake signals are rarely used by any GPS receiver that I know of or required (an exception is the DeLorme Earthmate).
This means that a simple two-wire connection is sufficient for all common applications: GND and TxD (gps), for certain configuration tasks also RxD (gps), then a three-wire connection.
With the GPS receiver you have to get the pin assignment of the RS232 interface. If this is designed as a SUB-D9 connector, you can usually assume a so-called DTE device, i.e. the computer and GPS receiver can be connected via a normal 1: 1 RS232 cable. The socket on the receiver represents, like a modem, an RS232 'input'.
Some GPS receivers can be configured as DCE devices, in this case the connector represents an RS232 'output'. A crossed or null modem cable must be used to connect to a computer. In this case the TX and RX connections in the cable are connected crosswise. This ONLY refers to the standardized pin assignment on the SUB-D connector. Of course, depending on the respective device, TxD of the GPS receiver is ALWAYS connected to RxD of the computer and RxD of the GPS receiver to TxD of the computer. You always play it safe with the exact pin assignment of the GPS receiver. If you are unsuccessful with standard cables + possibly adapters, you should rather solder the cable yourself using the pin assignments. A two-wire cable with GND and Tx (gps) -> Rx (pc) is sufficient for the first connection. For this you can e.g. use a 1-core shielded cable, as is usual for simple stereo audio connections. For the version with Rx (gps) -> Tx (comp) you need a two-core shielded cable, something like this is also common for audio connections. In this case, the Tx and RX signals are connected via the actual inner conductor of the cable, the GND signals via the shield of the cable. If the GPS receiver has separate GND / Logical GND and ground / shield connections at its interface, these should also be connected separately. In this case, GND / Logical GND should be transmitted via an (additional) inner conductor, and ground / shielding via the cable shield. Most of the time, the two are connected anyway.
In the case of short test connections, the type of cable is irrelevant, you can use any cable, even without shielding.
Cable assignment GPS receiver -> standard RS232 interface DSUB9 / DSUB25, as is commonly used on IBM compatible PCs and many other computers
|Tx (gps)||Rx (pc)||2||3|
|GND (gps)||GND (pc)||5||7|
|possibly also Rx (gps)||possibly also Tx (comp)||3||2|
Cable assignment GPS receiver -> RS422 interface, as used on Apple Mac computers. On some computers it is also called GEOPORT.
|GPS receiver||Apple Mac||Pin|
|Tx (gps)||Rx (mac)||5|
|GND (gps)||GND (mac)||4/8|
|possibly also Rx (gps)||possibly also Tx (mac)||3|
Cable assignment GPS receiver -> RS232 interface / HotSyncPort for PalmPilots
|Tx (gps)||Rx (pilot)||3|
|GND (gps)||GND (pilot)||10|
|possibly also Rx (gps)||possibly also Tx (pilot)||5|
The PalmPilots work with RS232 levels of only + -6V. But this has no meaning for all GPS receivers I know.
Step 3: The first connection
If you want, you can now start your favorite GPS application software on the computer and wait to see whether the GPS signal has an effect.
Possibly. the application must be informed of the COM port connection (PC) or modem / printer port (Apple) to which the GPS receiver is connected. U.u. the protocol type (usually NMEA) and the communication parameters (with NMEA typically 4800 baud, NoParity, 1 stop bit, or in short, 4800.8, N, 1) must also be communicated. In most programs, the GPS reception must also be activated explicitly, for example with a menu command 'Start GPS', or 'Enable GPS', 'Start Tracking', etc. pp. After that, at least a status message for GPS reception should appear somewhere on the screen such as 'Receiving GPS Data', 'Waiting for GPS Data' etc .. In the event of problems, an error message may also appear very quickly, e.g. 'Couldn't open Port' if the interface to the GPS receiver was configured incorrectly (e.g. on the mouse connection, or a modem), with Apple Macs the set port can also be assigned to the AppleTalk / LocalTalk protocol.
In this case, the port must be correctly configured in the GPS software via the configuration settings or released for use with the GPS receiver, for Macs e.g. by assigning AppleTalk to another interface, e.g. Ethernet, or deactivating AppleTalk in the selection / Chooser .
Under certain circumstances, the corresponding port can be occupied by fax (receiving) software or other applications (for PalmPilot users e.g. by the HotSync Manager). Even if no modem or no PalmPilot Cradle is connected to the computer, the port can still be used by the software. In this case, quit the relevant applications.
Very often, however, there is no communication with the GPS receiver on the first attempt. Anyone who knows what to do with the relevant configuration parameters of the GPS software can of course try them out, or simply consult the instructions / online help.
For diagnosis, it is recommended to first check the connection using Terminal program.
Terminal programs are hardly used or required nowadays by normal computer users. In the early days of data communication, they were mostly used to establish contact between two computers using a modem. Today, terminal programs are mostly used to establish a connection between a computer and a device to be configured via RS232, e.g. telephone systems, modems, network devices, or GPS receivers, as well as for error analysis.
A terminal program allows data or text input from the keyboard to be sent to the serial interface of a computer and data arriving at this interface to be displayed on the screen.
The terminal program does not care whether a modem, another computer or a GPS receiver is connected to this interface.
As a rule, the terminal program itself cannot do anything useful with the received data, except to output this data in text form on the screen. Since the NMEA output of a GPS receiver is a fairly readable text representation (so-called ASCII output), you can access the data of a GPS receiver directly with a terminal program without having to rely on the compatibility of more complex GPS software.
Analysis using a terminal program under DOS:
MS-DOS does not contain a terminal program. You either have to buy such a program (in the past it was often included as an accessory for analog modems), many DOS terminal programs are now also free of charge or can be obtained as shareware from the Internet or from mailboxes.
As a simple variant, however, you can also use a type of terminal for diagnosis with MS-DOS home remedies: the input / output redirector.
This makes it possible, for example, to redirect the data arriving at the serial interface from a GPS receiver to a file, which can then be viewed using the DOS command 'type' or the MS-DOS editor. To configure the receiver, keyboard entries from the screen can also be redirected to the serial output of the computer and thus to the GPS receiver.
The whole thing is somewhat unusual and stone-age for the pure user of graphical user interfaces, but quite possible. If no terminal program for DOS or Windows is available and cannot be installed later, it is the only option for low-level analysis.
First of all, you have to start MS-DOS or quit Windows under Windows 3.1 (
Two processes are necessary to record the GPS data in a text file:
- Configuration of the serial interface required by the GPS receiver to the correct communication parameters using the MODE command.
If the GPS receiver is connected to COM2 this would be:
MODE2 4800,8, N, 1
When connected to COM1 on the other hand
MODE1 4800,8, N, 1
If you are not sure which interface is COM1 or COM2 on the back of the computer, you can try both variants.
Older computers with AT boards usually have a COM interface with a 9-pin DSUB connector and one with a 25-pin DSUB connector. The 9-pin is mostly COM1, the 25-pin COM2. With newer ATX boards there are mostly two 9-pin DSUB plugs directly next to each other. Usually COM1 is the right / lower of the two sockets, COM2 the upper / left.
- Start the redirection to a file
After starting the redirection, wait a while, approx. 10s, and then cancel the redirection with
The file gps.txt then contains the GPS data received at the serial interface during this time; you can use it with
Analysis using a terminal program under Windows:
With all Windows versions, Win3.1, Win95, Win98, WinNT, a terminal program is included in the scope of delivery of the operating system. With WIN95 and WIN98 it may not have been installed on the hard drive of the computer. In this case it must be over -> START -> SETTINGS -> CONTROL PANEL -> SOFTWARE -> WINDOWS SETUP -> CONNECTIONS-> (Details) HYPERTERMINAL can be installed from the CDROM.
In Windows 3.1, the terminal is simply called terminal and is located in the 'Accessories' group or in the Windows directory under 'TERMINAL.EXE'.
The terminal program is called under WIN95 and WIN98 Hyperterminal and is usually located under -> START -> PROGRAMS -> ACCESSORIES -> HYPERTERMINAL -> HYPERTRM.EXE, or in the directory C: \ Programs \ Accessories \ HyperTerminal \ Hypertrm.exe.
You start HyperTerminal, after the start the program asks first for the name of the connection. Here you simply enter 'Test' or, if you want to use the configuration multiple times for analysis, a unique name, such as 'GPS Test'. Confirm with 'OK'. Then Hyperterminal asks for a phone number or offers a dial-up dialog via modem. Since we have connected a GPS receiver, we cannot do anything with it.Instead we select (scroll all the way down) in the lower selection bar 'Connect via:' the entry 'Direct connection via COMx', where x denotes the corresponding interface. When connecting the GPS receiver to COM2, we select 'Direct connection via COM2' and confirm with 'OK'.
A dialog for setting the communication parameters follows. For the usual NMEA protocol we choose here:
Bits per Second: 4800
Data bits: 8
Stop bits: 1
Protocol: None (this refers to the so-called handshaking, which, as already mentioned, is not used with GPS receivers)
The settings under 'Advanced' do not have to be taken into account.
After confirming the communication parameters, the input / output window, the actual terminal, appears. If you receive an error message 'COM x could not be opened', the corresponding interface is already being used by another application. So you should close all other programs and, if necessary, check all symbols in the right corner of the start bar for suspicious programs such as 'Fax Receive', 'HotSync', etc. and close them if necessary. If a mouse is connected to COM1, the GPS receiver can only be connected to COM2 (or vice versa).
If the connection works, the terminal window should already be filled with characters that scroll continuously across the screen. In this case you can at least be sure that data will arrive at the serial interface of the PC.
If nothing happens or the terminal window remains empty, the GPS receiver either does not send any data on its own, or the connection cable is incorrectly assigned or defective and must be checked. Before doing this, however, you should check the instructions for your GPS receiver to determine whether the transmission of data via the serial interface has to be specifically switched on. Possibly. the handshake setting of the terminal program must also be checked (NO HANDSHAKING).
If the GPS receiver sends data in the usual NMEA 4800,8, N, 1 format, the output in the terminal window looks like this:
$ GPRMC, 133626, A, 5336.432, N, 00957.436, E, 000.0,035.0,040897,000.3, W * 6C
$ GPGLL, 5336.432, N, 00957.436, E, 133627, A * 24
The GPS receiver usually sends its data sets continuously at fixed time intervals. With the NMEA protocol, each line represents a complete data set ('NMEA Message'), which is characterized by the character string following the $ character, i.e. the line with the preceding $ GPRMC contains a NMEA RMC message, the $ GPGLL a so-called GLL message . The different NMEA message types contain partly different data, partly the same. The GLL message contains e.g. only coordinates and time, whereas the RMC message contains coordinates, altitude above sea level, time, course, speed. Other messages contain information about the current satellite constellation, etc.
Each GPS receiver sends a different combination of NMEA messages, with some devices i
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