Sea-Bird Electronics SBE 38 Manual do Utilizador descarregar pdf (Página 15)

Section 4: Deploying and Operating RS-232 SBE 38

Power and Cable Length

There are two issues to consider:

• Limiting the IR loss to 1 volt; higher IR loss will prevent the

instrument from transmitting real-time data because of the difference in

ground potential.

• Supplying enough power at the power source so that sufficient power is

available at the instrument after considering IR loss.

Looking at each issue separately:

Limiting IR Loss to 1 Volt

The limit to cable length is typically reached when the maximum current times

the power common wire resistance is more than 1 volt, because the difference

in ground potential of the SBE 38 and ground controller prevents the SBE 38

from transmitting real-time data.

limit

= 1 volt = IR

limit

Where

I = current required by SBE 38 (15 mA for RS-232; 10 mA for RS-485).

Maximum cable length = R

limit

/ wire resistance per foot

Supplying Enough Power to SBE 38

Another consideration in determining maximum cable length is supplying

enough power at the power source so that sufficient voltage is available, after

the IR loss in the cable, to power the SBE 38.

Example 1 – For 18 gauge wire, what is maximum distance to transmit power to RS-232 SBE 38?

Is this controlling factor for maximum cable length if wanting to transmit at 1200 baud?

limit

= V

limit

/ I = 1 volt / 0.015 Amps = 67 ohms

For 18 gauge wire, resistance is 0.0064 ohms/foot.

Therefore, maximum cable length is 67 ohms / 0.0064 ohms/foot = 10417 feet = 3176 meters.

Note that 3176 meters > 800 meters (maximum distance SBE 38 can transmit data at 1200 baud), so IR loss is not

controlling factor for this example.

Example 2 – Same as above, but there are 4 SBE 38s powered from the same power supply.

limit

= V

limit

/ I = 1 volt / (0.015 Amps * 4 SBE 38s) = 17 ohms

For 18 gauge wire, resistance is 0.0064 ohms/foot.

Therefore, maximum cable length is 17 ohms / 0.0064 ohms/foot = 2604 feet = 794 meters (cable length to SBE 38

furthest from power source).

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 18 gauge wire, what is maximum distance to transmit power to RS-232 SBE 38 if using 8.5 volt power

source to supply power? Is this controlling factor for maximum cable length if wanting to transmit at 1200 baud?

SBE 38’s input power specification is 8 – 15 volts. Therefore, a 0.5 volt IR drop (8.5 volts – 8 volts) would still provide

enough power to SBE 38.

V = IR 0.5 volts = (0.015 Amps) * (0.0064 ohms/foot * cable length) Cable length = 5208 ft = 1588 meters

Note that 1588 meters > 800 meters (maximum distance SBE 38 can transmit data at 1200 baud), so IR drop in power is

not controlling factor for this example.

Example 2 – Same as above, but there are 4 SBE 38s powered from the same power supply.

V = IR 0.5 volts = (0.015 Amps * 4 SBE 38s) * (0.0064 ohms/foot * cable length)

Cable length = 1302 ft = 397 meters (cable length to SBE 38 furthest from power source).

Note that 397 meters < 800 meters (maximum distance SBE 38 can transmit data at 1200 baud), so IR drop in power is

controlling factor for this example. Using a higher voltage power supply or a different wire gauge (12 or 14 gauge) would

provide sufficient power at SBE 38 to allow 800 meter cable length.

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