Manual

June 2004 Isis® Sonar User's Manual, Volume 2 109
F.2.2 Theory of Operation
The trigger reference and up to four synchronous channels are passed through the SIU4.
The trigger is detected and conditioned in the SIU4 and embedded in the Channel 1
(usually port side-scan) signal. The trigger input is AC-coupled to sense the mean voltage
level of the trigger signal and shift it to 0 V. Any trigger pulse variance of 1.1 V or larger in
either direction from the detected mean level will trigger a dual, monostable multivibrator.
The first monostable controls the trigger holdoff, blocking any further triggers until it has
timed out at 10 µs, 25 µs, or 125 µs. This holdoff feature inhibits multiple triggering from
the falling edge of the input trigger, remnants of the sonar transmission cycle, and
immediate acoustic returns. The second monostable sets the output trigger pulse width at
1.0 µs. A solid state switch is used to embed the 1.0 µs output trigger in the channel 1
signal. All trigger and channel inputs are diode protected to
+6.0 V.
Solid state switches are also used to connect the appropriate series attenuation resistor
for each channel based on its rotary switch setting. The rotary switches are located under
their respective channel output BNC. The switch positions are labeled 0 to 9 and
correspond to 0% to 90% signal attenuation based on the 10 kΩ load. The DSP C31
analog card presents a high impedance (> 1 MΩ) load, so 10 kΩ load resistors are fitted
on the SIU4 printed circuit board for this situation. See “2.4.4 Setting the SIU4 Signal
Attenuation”.
The embedded trigger output can equal either DC power level (+5 V or -5 V) by setting
jumper J1. The default is -5 V. All outputs are unprotected except in the Isis (32C) version
where they are clipped at
+ 2.5 V because of the input sensitivity of the Isis 32C A/D–
DSP card.
Several functional attributes of the SIU4 are defined by jumper settings on the PCB.
These settings are shown in Table F-2.
Setting jumpers on a SIU4 is more than just a matter of making the SIU4 operational.
Different jumper settings can optimize the SIU4’s triggering to be used with a given
sonar. Synchronizing the pulse rate of the trigger to the sonar’s range will enable Isis to
collect the maximum amount of data during the survey.
Appendix F: Working with Signal Interfaces