Principles of Operation:
Hardware

This section is a catch-all for hardware issures, containing principles, background concepts, specifications and recommendations. The following pages go into detail on specific topics:

1. Pressure sensor
2. Compass calibration
3. Analog inputs
4. Lithium battery packs
5. Anti-fouling
6. Frequently asked questions (below)

Frequently Asked Questions

1. What is the difference between RS422 and RS232?
2. How can I ensure success when I download large datasets?
3. What makes serial communication through radio modems different?
4. How does an Aquadopp use power?
5. How does adjustable transmit power work?

1. What is the difference between RS422 and RS232?  

The main difference is that RS422 needs more wires, and it transmits data more reliably.

RS232 uses a single ground wire, plus one wire each for transmitting data and receiving data. RS422 uses a ground wire plus pairs of wires for transmitting and receiving data. RS422 then uses the voltage difference across the pair for data transmission. Then, since each wire tends to pick up the same noise, the noise is cancelled when the circuit looks at the voltage difference. This process, called common-mode noise rejection, enables data transmission over long distances without data corruption.

2. How can I ensure success when I download large datasets?  

Downloading data now is easier than it used to be because Nortek instruments check each block of data for int6egrity. If a block is bad, the software askes for the block again. Our experience is that modern Windows PCs are also able to handle downloads at 115,200 baud, but we have also seen that data downloads can be a frustrating problem as well.

If you have trouble downloading data, try the following:

• Disable power management.
• Upgrade your software (and if necessary, your firmware) to the latest version.
• Disable background routines. FindFast (see below) has been a problem in the past.
• Quit all programs other than the one you are using for the download.
• Try a lower baud rate.
• Try a different computer.

Power management is the most serious problem people seem to have now. If power management takes over the PC, data downloads can be disrupted sufficiently to cause the process to fail. Power management is most commonly enabled in laptop computers, but it can be enabled in desktops as well.

The other problems are less likely to cause trouble now because the software checks each block of data, and if a block is corrupted in any way, it asks the instrument to retransmit the block. If the program must retransmit too many blocks, it can slow down. Then consider items 2-6 of the list above.

To disable Find Fast in Windows98, use the following steps:

• In the Control Panel, double-click Find Fast.
• On the Index menu, click Close and Stop.
• Find Fast stops running until you restart the computer, or you open Find Fast again from the Control Panel.

3. What makes serial communication through radio modems different?  

The main difference between radio modems and wires is that most (or all?) radio modems are unable to transmit hardware breaks. When this happens, you must use "software breaks" to control an instrument.

Serial data transmission causes the serial data line to switch back and forth between two different voltages. The rate at which it switches back and forth depends on the baud rate of line, but it is certain that the voltage will switch back and forth many times in a second. A hardware break looks very different from normal data communication because the voltage switches and stays that way for a relatively long time. Instruments commonly use special circuitry to detect a hardware break as a means to trigger wake up or to get attention.

When you cannot transmit a hardware break, instruments can substitute different circuitry to detect a special string of characters. This special character string is called a software break.

4. How does an Aquadopp or EasyQ use power?  

An Aquadopp has four power states: transmit, operate, wait and sleep.

While collecting data, an Aquadopp alternately transmits sound and processes the echo. If the measurement load is 100%, it continues to do this until the averaging interval is complete. If the measurement load is less than 100%, then the Aquadopp goes into a lower-power wait state during part of each second. For example, an Aquadopp Current Meter pings 69 times every second, or 23 times for each beam. An 8% measurement load means that it will ping each beam 2 times then go into wait state for the rest of each second. A lower measurement load allows the Aquadopp to spread out its averaging interval over a longer period of time

If the measurement interval is longer than the averaging interval, the Aquadopp goes to sleep for the remainder of the measurement interval. Sleep energy consumption is far lower than for the other states.

5. How does adjustable transmit power work?  

An Aquadopp can adjust its power over four levels in roughly 6 dB steps.

A difference of 6 dB is about a factor of 4 in power. At the lowest level, the Aquadopp uses little energy for transmit, while at the highest level, most of the energy consumed by the Aquadopp goes to the transmit pulse. Higher transmit power allows the Aquadopp to work in lower backscatter, but it causes the battery to deplete faster.