User`s manual

Manual revision 016 Section 4: Deploying and Operating FastCAT SBE 49
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Power and Cable Length for FastCAT without Deck Unit
On the ship, cables longer than 3 meters should be installed inside an earthed
metal conduit by a qualified electrician. This minimizes the potential for
external signals to disrupt communication and ensures that high voltage lines
(such as the sea cable) are sufficiently protected. Cables shorter than 3 meters
can be used without shielding when installing or bench testing the instrument.
There are two issues to consider:
• Limiting the communication IR loss to 1 volt if transmitting real-time
data; higher IR loss will cause the instrument to transmit data that does
not meet the RS-232 communication standard.
• Supplying enough power at the power source so that sufficient power
is available at the instrument after considering IR loss from the
turn-on transient.
Each issue is discussed below.
Limiting IR Loss to 1 Volt
The limit to cable length is typically reached when the maximum current
during communication times the power common wire resistance is more than
1 volt.
V
limit
= 1 volt = IR
limit
Maximum cable length = R
limit
/ wire resistance per foot
where I = current required by FastCAT during communication.
The value for I varies, depending on the input voltage –
• 390 mA at 9 V
• 285 mA at 12 V
• 180 mA at 19 V
See Specifications in Section 2: Description of FastCAT.
Supplying Enough Power to FastCAT
Another consideration in determining maximum cable length is supplying
enough power at the power source so that sufficient voltage is available, after
IR loss in the cable (from the 0.75 Amp turn-on transient, two-way
resistance), to power the FastCAT. Provide at least 9 volts, after IR loss.
V - IR > 9 volts
where I = FastCAT turn-on transient (0.75 Amps; see Specifications in
Section 2: Description of FastCAT).
Example – For 12 volt power supply and 20 gauge wire, what is the maximum distance to transmit power to the FastCAT
when considering communication IR loss?
At 12 V, I = 0.285 Amps. For 20 gauge wire, resistance is 0.0107 ohms/foot.
R
limit
= V
limit
/ I = 1 volt / 0.285 Amps = 3.5 ohms
Therefore, maximum cable length is 3.5 ohms / 0.0107 ohms/foot = 327 feet = 100 meters.
Note that 100 meters < 800 meters (limit to distance FastCAT can transmit data at 1200 baud, as shown in example in
Baud, Cable Length, and Data Transmission Rate), so cable is limited to 100 meters because of power constraints.
Alternatively, you can transmit data over 800 meters but provide a power source closer to the FastCAT.
Note:
Common wire resistances:
Gauge Resistance (ohms/foot)
12 0.0016
14 0.0025
16 0.0040
18 0.0064
19 0.0081
20 0.0107
22 0.0162
24 0.0257
26 0.0410
28 0.0653
Example 1 – For 20 gauge wire, what is maximum distance to transmit power to FastCAT if using 12 volt power source?
V - IR > 9 volts 12 volts - (0.75 Amps) * (0.0107 ohms/foot * 2 * cable length) > 9 volts
3 volts > (0.75 Amps) * (0.0107 ohms/foot * 2 * cable length) Cable length < 187 ft = 57 meters
Note that 57 meters < 100 meters (maximum distance when considering communication IR loss), so supplying enough
power during power-on transient is controlling factor for this example. Using a different wire gauge would increase
allowable cable length.
Example 2 – Same as above, but there are 4 FastCATs powered from same power supply.
V - IR > 9 volts 12 volts - (0.75 Amps * 4 FastCATs) * (0.0107 ohms/foot * 2 * cable length) > 9 volts
3 volts > (0.75 Amps * 4 FastCATs) *(0.0107 ohms/foot * 2 * cable length)
Cable length < 46 ft = 14 meters (to FastCAT furthest from power source)