BNC 130 - Manual (Page 17)

17
SECTION 3
THEORY OF OPERATION
GENERAL
Module Interface
Figure 3-1 shows a simplified block diagram of the Module Interface board. The path for communication
between mainframe and module is via PS. The eight QAD lines and five QA lines are the bus interface
lines, and a MOD DIS line is used for disabling the Output board. Power is also delivered by P8.
The address demultiplexer and select logic circuitry decodes the bus signals and selects one of the other
blocks.
The I.D. ROM contains information necessary for operation specific to the Output board. This includes
boundaries for parameters, values used to initialize the nonvolatile RAM (NVRAM), and the version
number of the I.D. ROM.
The nonvolatile RAM is used for saving and retrieving ten panel settings. It also holds the GPIB/RS232
bus settings.
The digital control circuits are used to monitor and set the operating state of the Output board. The DACs
and amplifiers allow setting of up to four analog values used for level control. All of the control, status,
and analog signals are delivered to the Output board by two 20-pin connectors, J1 and J2.
Laser Driver Output
Figure 3-2 shows a simplified block diagram of the Laser Output board. The connection to the Module
Interface board is shown in the lower left hand corner. There are three analog lines used to set the output
levels and the threshold of the External Drive discriminator. The control lines select which Mode the
Laser Output will operate in.
The selection of the digital drive source is controlled by three digital control lines, EXT DRIVE ENABLE,
IMPULSE, and EXT MOD. These determine which of the two drive sources will be controlling the state of
S1, (a high speed transistor switch). A discriminator, whose threshold is set by the analog voltage EXT
DRIVE LEVEL, is connected to the drive source from the front panel EXT DR connector. The other signal
source is the rear panel DRIVE connector which delivers the mainframe's pulse generator output.
The level control circuitry sets one or both of the current sources as determined by the analog inputs
PEAK LEVEL, and BASELINE LEVEL and the Mode control lines. During the Impulse, CY and External
Modulation Modes, it uses the PIN detector in the laser module to monitor and stabilize the laser's
output. When LASER GUARD is active, the current sources are set for zero output.