Using Workhorse ADCPs with Long Cables - SF

Using Workhorse ADCPs with Long Cables - SF
RD Instruments
Application Note
Acoustic Doppler Current Profilers
FSA-021 (November 2004)
Using Workhorse ADCPs with Long Cables
1
Introduction
RS232 communications is limited to a maximum cable length of 15 meters
(49.2 feet). To ensure maximum communication reliability, the RS422
communication mode must be used for cables longer than 15 meters. All
Workhorse ADCPs have a RS232/422 converter built in. Changing the
communication mode between RS232 and RS422 is made with the white
switch located on the top of the electronics (there is only one switch in the
system).
NOTE. RDI Workhorses do NOT use control signals. Only three wires are
needed to communicate in RS232 mode and five wires are necessary to
use RS422.
2
RS-232 to RS-422 Converters
RD Instruments provides the BB Electronics’ Model 422LP25R PortPowered RS-232 to RS-422 Converter. This is a port-powered converter
and some precautions must be taken when using this kind of converter:
• The converter gets its power from the RS232 signal, therefore the
RS232 cable should be kept as short as possible.
• All RS232 lines from the computer to the converter should be wired.
Figure 1.
BB Electronics’ Model 422LP25R Port-Powered RS-232 to
RS-422 Converter
NOTE. This converter may not work at all with laptops running on batteries
(some laptops reduce the voltage on RS232 lines to save power). See the
reference section to find more info about port-powered converters.
FSA-021 (November 2004)
page 1 of 4
Using Workhorse ADCPs with Long Cables
2.1
General Points About Communications
It is always preferable that the communication device uses the same power
supply and ground as the input power supply. With externally-powered
converters, it is not easy to control ground potential. This is why it is
strongly recommended to use optically isolated conveters for permanent
installations or installations using long cables.
A few customers have successfully used the 485OT9L or the 485OTLED
models from BB Electronics. See the reference section for the data sheet.
2.2
Power Requirements
The Workhorse Monitor/Sentinel requires a DC supply between 20 and
60 volts. The power supply must be able to handle the inrush current as
well. Inrush current is the current required to fully charge up the capacitors
when power is applied to the ADCP. The capacitors provide a store of energy for use during transmit. The inrush current is as high as 3 Amps rms.
The ADCP will draw this amperage until its capacitors are fully charged.
If the power supply limits the current or the power drop on the cable is significant, then the power on cycle will take longer. It can take up to one minute. You do not want the power to shut down during the inrush current
draw, as this may not allow the ADCP electronics to start.
2.2.1
Power Supply
The power supplies provided by RD Instruments with the system are designed to maintain a 400-ma supply under this load. They are 75-Watt supplies, with 48 VDC, 1.5 amp outputs. They will not fall back to 0 amps, 0
volts under a load. Customer provided power supplies might shut themselves down under such a load; when that occurs, the Workhorse will not
wakeup.
It is always recommended that the system be powered with the maximum
input voltage allowed. Transmitted power increases or decreases depending
on the input voltage (within the voltage range of 20 to 60 VDC for the
WorkHorse Monitor/Sentinel). The transmitted power is decreased 1 DB if
the input voltage drops from 42 VDC to 33 VDC. For a 300kHz WorkHorse ADCP, each DB will result in a decrease in range of one default
depth cell.
Workhorse Monitors and Sentinels operate at a maximum input voltage of
60 VDC, while Workhorse Long Rangers use up to 50 VDC. The recommended minimum input voltage is 48 Volts.
page 2
RD Instruments
RD Instruments
Acoustic Doppler Current Profilers
Application Note
FSA-021 (November 2004)
2.2.2
Adding a Battery Pack
Connecting a battery pack at the Workhorse-end of the cable is the preferred
method to overcome the inrush current requirements and power drop on the
cable. You can do this with either an internal battery installed in the Sentinel, or a Monitor with an external battery case. The battery acts as a backup
power source for the external power and will help at start-up. As an added
benefit, the internal Workhorse memory can be used during cable maintenance operations since the ADCP is still powered by the battery.
The power supply input voltage must be sufficient to override the internal
battery voltage. For example, the ADCP will draw all power from the
power supply even if the battery is installed and connected under normal
load conditions. During startup, if the power supply voltage drops below
the voltage level of the battery, the ADCP will draw current from the battery. Once the voltage level rises above that of the battery, the ADCP will
resume drawing current from the external power supply. A fresh battery
provides +42 VDC. Batteries spend most of their life at a nominal voltage
of +33 VDC.
NOTE. There have been some cases where, during each waves burst, the
cable can’t provide all of the needed power for the system. Consequently,
the Workhorse draws some power from the battery at each burst. After
some time, the battery goes low and the system lacks power during burst.
Symptom: the system starts a waves burst but never finishes it.
WavesMon shows “Reading data: discontinuity” message.
2.3
Choosing the Cable
RDI recommends two cable suppliers:
• Rochester Cable, reference 7-H-422A or equivalent.
http://www.rochestercables.com
• MacArtney
http://www.macartney.com
FSA-021 (November 2004)
page 3 of 4
Using Workhorse ADCPs with Long Cables
2.4
Cable Termination
Splicing of the cable is one of those procedures that looks simple but is best
performed by an expert; otherwise reliability may be a problem. We often
work with a company that supplies cables to your specified length, assembled and tested.
• PMI Industries Galvanized Steel Cable Grip PMI 2202-7525 or equivalent, two required for strain relief at each end.
• PMI Industries DAM/BLOK Splice Kit PMI 2207-7069 or equivalent,
one required for connecting armoured cable to underwater connector.
http://www.pmiind.com/
RDI can provide a pigtail cable, with WH matching connector at one
end and non-terminated at the other end. Reference: WHPIG
WH standard bulkhead connector is an LPMBH-7-MP from Impulse.
High-pressure systems use LPMBH-7-MP-1; 12 Volts systems use
LPMBH-8-MP (Except Channel Master).
http://www.impulse-ent.com/pdfs/LPMIL_LPMBH.pdf
3
References / Technical Information
• General Information about Communication Systems
http://www.bb-elec.com/technical_library.asp
• Why RS422?
http://www.bb-elec.com/tech_articles/connection_illustration.asp
• Choosing a Cable for RS422 Communication
http://www.bb-elec.com/tech_articles/cable_selection.asp
• RDI port powered RS422/232 Converter; 422LP25R specification sheet
can be found at:
http://www.bb-elec.com/bb-elec/literature/422lp25r-3903ds.pdf
• Tips for Using a Port Powered Converter
http://www.bb-elec.com/tech_articles/port_powered_converter_tips.asp
• Using a Port Powered Converter
http://www.bb-elec.com/tech_articles/port_powered_converter_faq.asp
• How optical Isolation Works
http://www.bb-elec.com/tech_articles/optical_isolation_illustration.asp
• Choosing an Optically Isolated Converter
http://www.bb-elec.com/product_family.asp?FamilyId=14
• RDI “Standard” cable; Belden cable reference 8427
http://bwcecom.belden.com/college/Cable101/Cndcfgtp.htm
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RD Instruments
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