Is there anybody who could answer me a question concerning NMEA2000
priority settings?
There are only 3 bit available for the priority of a message.
Collissions are anyhow avoidid, because the afterwards folloing bits
are those of the PGN (Parametric Group Number), which is unic for
every message. Are the messages sorted by their priority and then
given their PGN? If yes, how about adding messages in later versions?
If no, how to come along with 8 different priorities? How to manage
that the most important message gains the access on the bus?

Many questions, I know. Hoping for many answers. Thanks a lot!
Sabine

Sabine --

Perhaps this may help.

NMEA 2000(R) is based on the CAN (Controller Area Network) standard
developed by Bosch Gmbh. CAN uses either 11 bit or 29 bit identifier fields
for arbitration, each time a frame is sent on the network - if two or more
devices attempt to transmit simultaneously, each device listens to each bit
it transmits and if it detects a bit that is different than the one its is
transmitting, then it stops its transmission immediately. Since the zero
value is dominant on the network, the device with the lowest identifier
wins.

With NMEA 2000(R), the identifier field is composed of (from most
significant to least) a 3 bit priority, a reserved bit, a 17 bit Parameter
Group Number (PGN), and an 8 bit source device address. NMEA 200(R) uses 29
bit identifiers exclusively. PGNs identify the group of parameters and the
format for the data that will follow in the frame.

PGN numbers are assigned so that more important values have lower PGNs than
less important values, for example, "Rudder" is PGN 127245 and "Moored Buoy
Station Data" is PGN 130324. If these two PGNs were transmitted at the same
time, the PGN containing the rudder angle data would succeed before the PGN
containing information about a moored buoy. Each PGN also has a default
priority associated with it that a manufacturer is supposed to use in the
priority bits, unless some application specific reason exists to do
otherwise. The ability to modify the priority bits give some flexibility to
the system designer to account for unique situations.

The device address only plays a role when the same PGN (with the same
priority) is being transmitted simultaneously from two different devices.
In this case PGN from the lowest addressed device succeeds first. Again, a
mechanism is employed to ensure that more important devices get lower
addresses than less important devices.

The important thing to remember, is that the PGNs and default priorities are
not assigned arbitrarily, but are assigned in accordance with some measure
of their relative importance in maritime operations. Enough room remains in
the specification so that messages added in later versions can be assigned a
PGN that will position the PGN appropriately relative to other PGNs. In the
event that a specific application arises where a higher priority is
justified, then the priority bits can be changed to accommodate the
situation.

Dave
MV "Setag"
dave_at_netsavvy_dot_com_remove_'s

"Sabine" wrote in message
m...
Is there anybody who could answer me a question concerning NMEA2000
priority settings?
There are only 3 bit available for the priority of a message.
Collissions are anyhow avoidid, because the afterwards folloing bits
are those of the PGN (Parametric Group Number), which is unic for
every message. Are the messages sorted by their priority and then
given their PGN? If yes, how about adding messages in later versions?
If no, how to come along with 8 different priorities? How to manage
that the most important message gains the access on the bus?

Many questions, I know. Hoping for many answers. Thanks a lot!
Sabine