User`s manual

Manual revision 016 Section 4: Deploying and Operating FastCAT SBE 49
25
Baud, Cable Length, and Data Transmission Rate
Without a Sea-Bird Deck Unit
The rate that real-time data can be transmitted from the FastCAT is dependent
on the amount of data to be transmitted per scan and the serial data baud rate:
Time required to transmit data =
(number of characters * 10 bits/character) / baud rate
where
Number of characters is dependent on included data and output format
(see Data Output Formats). Add 2 to the number of characters shown in
the output format, to account for the carriage return and line feed at the
end of each scan. For decimal output (OutputFormat=2 or 3), include
decimal points, commas, and spaces when counting characters.
Time required to transmit data must be less than the real-time output rate.
Minimum baud rates are tabulated below for 16 Hz sampling (NAvg=1) and
8 Hz sampling (NAvg=2): (see Examples below for calculation examples)
NAvg
OutputFormat
Minimum Baud
1
0 or 1
4800
2 or 3*
9600
2
0 or 1
2400
2 or 3*
4800
* Including output of salinity and sound velocity.
The length of cable that the FastCAT can drive to transmit real-time data is
also dependent on the baud rate. The allowable combinations are:
Maximum Cable Length (m)
1600
800
400
200
100
50
25
Maximum Baud
600
1200
2400
4800
9600
19200
38400
With a Sea-Bird Deck Unit
Set the FastCAT baud rate to 4800 if using the FastCAT with either of the
following real-time data acquisition systems:
• SBE 36 CTD Deck Unit and PDIM
• SBE 33 Carousel Deck Unit and SBE 32 Carousel Water Sampler OR
SBE 33 Carousel Deck Unit and SBE 55 ECO Water Sampler
The data telemetry link can drive 10,000 meters of cable while accepting
4800 baud serial data. The relationship between transmission rate, amount of
data transmitted, and baud rate is as described above for a FastCAT without a
Deck Unit.
Notes:
• Baud rate is set with Baud=.
• Real-time output rate is set with
NAvg=.
• Output format is set with
OutputFormat=.
See Command Descriptions.
Example 1 - What is fastest rate you can transmit real-time data over 800 m with OutputFormat=0 (raw hex data)?
With 800 meters of cable, the FastCAT requires a baud rate of 1200 or less.
Number of characters for OutputFormat=0 (from Data Output Formats) =
6 (T) + 6 (C) + 6 (P) + 4 (P temperature compensation) + 2 (carriage return & line feed) = 24
Time required to transmit data = (24 characters * 10 bits/character) / 1200 = 0.2 sec > 0.0625 sec (16 Hz, maximum sampling rate).
Therefore, set NAvg=4, averaging 4 measurements/sample and transmitting 1 sample every 0.25 seconds.
Example 2 – Same as above, but OutputFormat=3 (engineering units in decimal), and OutputSV=Y and OutputSal=Y (calculated
salinity and sound velocity along with pressure, temperature, and conductivity).
Number of characters for OutputFormat=3 (from Data Output Formats) =
8 (T) + 2 (comma & space) + 8 (C) + 2 (comma & space) + 8 (P) + 2 (comma & space) +
8 (salinity) + 2 (comma & space) + 8 (sound velocity) + 2 (carriage return & line feed) = 50
Time required to transmit data = (50 characters * 10 bits/character) / 1200 = 0.42 sec > 0.0625 sec (16 Hz, maximum sampling rate).
Therefore, set NAvg=7, averaging 7 measurements/sample and transmitting 1 sample every 0.4375 seconds.
Example – FastCAT with SBE 36 and PDIM. What is fastest rate you can transmit data over 800 m with OutputFormat=0 (raw hex
data, which is the required output format for use with the Deck Unit)?
With a Deck Unit, the FastCAT requires a baud rate of 4800.
Number of characters for OutputFormat=0 (from Data Output Formats) =
6 (T) + 6 (C) + 6 (P) + 4 (P temperature compensation) + 2 (carriage return & line feed) = 24
Time required to transmit data = (24 characters * 10 bits/character) / 4800 = 0.05 sec < 0.0625 sec (16 Hz, maximum sampling rate).
Therefore, set NAvg=1, providing 16 Hz data (1 sample every 0.0625 seconds).