NMEA-0183 Interface: Signal Data Rate; Data to be Exchanged; TALKER ID
Posted: Sat Apr 09, 2016 9:22 am
This article is an extension to and a companion to my NMEA-0183 Data Collection article at
viewtopic.php?f=9&t=816
Signal Data Rate
Most discussion of NMEA-0183 interconnections focuses on the PHYSICAL layer of the interface of two devices using NMEA-0183. It is in the physical layer that most problems occur, and they occur due to improper physical connection of the signals. But there are two more layers of the connection can also affect the outcome. The signal data rate is often able to be controlled by the operator in the set-up of each device. The signal data rate has to match between devices, otherwise their interfaces won't communicate. The operator has to configure the two devices being interfaced so they have a common data signal rate.
In NMEA-0183 the signal rate is usually pre-set to be 4800-baud. This is the standard rate, and generally all devices are configured with this rate by default. In some cases it may be necessary to change to the high-speed rate of 38400. This is a special case and will be discussed in another article in this thread. (See below.)
The Data to be Exchanged
Once the physical connection has been properly made and the device's interfaces are set to a common signal rate, communication is going to occur between devices. But there is one more layer in the communication system or protocol that must be configured properly: the data that is going to be sent across this communication link has to be properly configured. In the NMEA-0183 protocol there are provisions for sending all kinds of data about all sorts of parameters related to a boat. In the specific case of interconnecting a GNSS receiver to a DSC radio, the radio only wants to hear information about the boat's current position. The radio won't know what to do with other information, it just wants information about the boat's position.
In the NMEA-0183 protocol, data is sent in specified formats called sentences. The structure of these sentences is very specific. There are maybe one hundred sentences possible to send. Each one of those sentences contains certain specific information. Each radio has been designed to listen for a specific sentence or a group of several sentences that will contain the boat position information. In order to get any data to be passed across the communication link, the sending device has to send the data in a sentence that the receiving device can understand.
To know what NMEA-0183 sentence a device can send or understand, we again rely on the manufacturer of the device to tell us this information. Somewhere in the literature provided with every NMEA-0183 device there will be a list of the NMEA-0183 sentences the device can send and can receive. Once a sentence is found that is in common with both devices, then those two devices can communicate that sentence and the information contained in the sentence.
Chart Plotter to Radio for GNSS Position Data
In the case of the interface of a chart plotter and a DSC radio, the information to be communicated is the boat position. The chart plotter usually has this information because it has its own GNSS receiver or there is an external GNSS receiver attached to it.
In the NMEA-0183 protocol there are several sentences which will include the boat position, so there are several ways of sending the boat position across a NMEA-0183 link. Usually a chart plotter can send all of these different sentences that will have boat position in their data. Some DSC radios are similarly flexible about that sentence they can get the boat position data from. Those radios know how to listen for four or five different sentences and can extract the boat position data from any of them.
Not all chart plotters and not all radios, however, can send or receive every possible NMEA-0183 sentence in the protocol. It is more common that each device can only send or receive a small subset of sentences. And even in the smaller group of sentences that just have boat data, it is possible that between two devices there may only be one or two of those sentences that are in common.
It is also typical that a device that is sending data does not send every possible sentence it knows how to send all the time. The device will only send the sentences that the operator configures it to send. In some instances the manufacturer may have set a default condition so their device will, right out of the box, send a few common sentences without any further intervention by the user. But it is also possible that the device is going to do nothing and will wait for the user to configure what sentences it will send.
On the device receiving information, the usual configuration is the device is ready to receive any of the sentences it can all the time. No configuration is needed.
Default Behavior Might Work
As a result of the default behavior to two typical devices, it may be possible to just make the physical connection between them and have immediate communication occur of the information desired. But, it is also possible this won't happen all by itself. The operator may have to configure the device sending the data, and set that device to send a specific NMEA-0183 sentence. And it may happen that the device receiving the data is very fussy about which one--of several possible sentences that might contain the data--it wants to listen for. In situations like this the installer must configure the sending device to use the specific sentence the receiving device knows how to use.
Exactly where in the literature for any device you will find the list of possible sentences it can send or receive is impossible to say. The installer just has to carefully read the literature for a device, and find the list of sentences that each device can send or receive. Then the installer selects one (or more than one) sentence that the two devices have in common.
Exactly where in any device you will find the configuration screens is impossible to say. The installer just has to be familiar with the user-interface of a device, and find the configuration portion. In the configuration, the installer selects what sentence or sentences are to be sent.
viewtopic.php?f=9&t=816
Signal Data Rate
Most discussion of NMEA-0183 interconnections focuses on the PHYSICAL layer of the interface of two devices using NMEA-0183. It is in the physical layer that most problems occur, and they occur due to improper physical connection of the signals. But there are two more layers of the connection can also affect the outcome. The signal data rate is often able to be controlled by the operator in the set-up of each device. The signal data rate has to match between devices, otherwise their interfaces won't communicate. The operator has to configure the two devices being interfaced so they have a common data signal rate.
In NMEA-0183 the signal rate is usually pre-set to be 4800-baud. This is the standard rate, and generally all devices are configured with this rate by default. In some cases it may be necessary to change to the high-speed rate of 38400. This is a special case and will be discussed in another article in this thread. (See below.)
The Data to be Exchanged
Once the physical connection has been properly made and the device's interfaces are set to a common signal rate, communication is going to occur between devices. But there is one more layer in the communication system or protocol that must be configured properly: the data that is going to be sent across this communication link has to be properly configured. In the NMEA-0183 protocol there are provisions for sending all kinds of data about all sorts of parameters related to a boat. In the specific case of interconnecting a GNSS receiver to a DSC radio, the radio only wants to hear information about the boat's current position. The radio won't know what to do with other information, it just wants information about the boat's position.
In the NMEA-0183 protocol, data is sent in specified formats called sentences. The structure of these sentences is very specific. There are maybe one hundred sentences possible to send. Each one of those sentences contains certain specific information. Each radio has been designed to listen for a specific sentence or a group of several sentences that will contain the boat position information. In order to get any data to be passed across the communication link, the sending device has to send the data in a sentence that the receiving device can understand.
To know what NMEA-0183 sentence a device can send or understand, we again rely on the manufacturer of the device to tell us this information. Somewhere in the literature provided with every NMEA-0183 device there will be a list of the NMEA-0183 sentences the device can send and can receive. Once a sentence is found that is in common with both devices, then those two devices can communicate that sentence and the information contained in the sentence.
Chart Plotter to Radio for GNSS Position Data
In the case of the interface of a chart plotter and a DSC radio, the information to be communicated is the boat position. The chart plotter usually has this information because it has its own GNSS receiver or there is an external GNSS receiver attached to it.
In the NMEA-0183 protocol there are several sentences which will include the boat position, so there are several ways of sending the boat position across a NMEA-0183 link. Usually a chart plotter can send all of these different sentences that will have boat position in their data. Some DSC radios are similarly flexible about that sentence they can get the boat position data from. Those radios know how to listen for four or five different sentences and can extract the boat position data from any of them.
Not all chart plotters and not all radios, however, can send or receive every possible NMEA-0183 sentence in the protocol. It is more common that each device can only send or receive a small subset of sentences. And even in the smaller group of sentences that just have boat data, it is possible that between two devices there may only be one or two of those sentences that are in common.
It is also typical that a device that is sending data does not send every possible sentence it knows how to send all the time. The device will only send the sentences that the operator configures it to send. In some instances the manufacturer may have set a default condition so their device will, right out of the box, send a few common sentences without any further intervention by the user. But it is also possible that the device is going to do nothing and will wait for the user to configure what sentences it will send.
On the device receiving information, the usual configuration is the device is ready to receive any of the sentences it can all the time. No configuration is needed.
Default Behavior Might Work
As a result of the default behavior to two typical devices, it may be possible to just make the physical connection between them and have immediate communication occur of the information desired. But, it is also possible this won't happen all by itself. The operator may have to configure the device sending the data, and set that device to send a specific NMEA-0183 sentence. And it may happen that the device receiving the data is very fussy about which one--of several possible sentences that might contain the data--it wants to listen for. In situations like this the installer must configure the sending device to use the specific sentence the receiving device knows how to use.
Exactly where in the literature for any device you will find the list of possible sentences it can send or receive is impossible to say. The installer just has to carefully read the literature for a device, and find the list of sentences that each device can send or receive. Then the installer selects one (or more than one) sentence that the two devices have in common.
Exactly where in any device you will find the configuration screens is impossible to say. The installer just has to be familiar with the user-interface of a device, and find the configuration portion. In the configuration, the installer selects what sentence or sentences are to be sent.