Computer Control
The
computer control interface is composed of a few, simple commands that
allow the user to:
Select the channel that is to be used as the preset channel
Specify the preset count or time
Specify the analog threshold for the front panel and rear panel
input signals
Specify the counter signal active edge – rising or falling
Specify the polarity of the Shutter Control Signal (active high or
active low)
Specify the state of the shutter signal (On or Off)
Start counting.
Pause Counting
Resume Counting
Stop Counting
Query the contents of the counters and timer (these can be read
while the counters and timer are active).
Neutron
diffraction experiments are normally conducted using a neutron beam
monitor to control the duration of the experiment. The IA
CounterTimer/ShutterControl (CTS) is designed to provide control of
the duration of neutron diffraction experiments either by the preset
number of events recorded in a neutron monitor or by a preset time.
The CTS is a 1-wide NIM module with three BNC counter inputs and one
BNC control signal output. The C/T-S communicates with the host
computer via USB bus and appears to the host as a serial port. All of
the commands are transmitted as simple single character command
codes, some accompanied with data (such as thresholds or preset
counts) where appropriate.
Computer
Interface/Command Set
Internally
the CTS is an FPGA based Counter/Timer module with a USB 2.0
interface. The USB interface goes through a FTDI USART chip to form a
serial link between the FTDI chip (USB subsystem) and an Altera AVR
processor (ATMEGA329PV -10AU which implements the housekeeping for
the module including the DAC interface for setting the thresholds.
After
the device driver is installed, the CTS appears as two USB Serial
Ports. The first is the interface to the device. The second is an
artifact of the (general purpose) driver and is ignored. The serial
connection is 8 bits 1 stop bit, no parity and no flow control at
115200 baud. The device can be accessed by a simple terminal emulator
(HyperTerm, RealTerm, Ewan …) to experiment with the command set.
The commonly used USB interface commands are listed below. There are
additional commands, described in the manual, that are used for
configuring and querying the device flash memory and the FPGA.
Specifications
Count Capacity: 232
-1 = 4,294,967,295
Maximum Counting Rate:
25 MHz
Time Base: 50 MHz clock
with minimum time interval of 10 ms.
Time Base Accuracy:
Within +/- 0.0025% over the 0o-50o C instrument
operating range.
Preset Time/Counts: The
CTS can be programmed to stop counting when a preselected preset is
reached in the selected counter (Counter 1, 2 or 3) or in the timer.
Presets can be any value up to the maximum (232-1). If the preset is
0, or presets are disabled, the CTS will run umtil stopped by command
of the host computer.
Input discriminators:
All of the inputs have computer control discriminators with the
discriminator level Vt = 6.4453mV*DAC – 1650mv where DAC
is the setting of the (10 bit) DAC associated with the input.
Pulse Pair Resolution: < 20 ns
Input signal levels compatible with ECL, Fast NIM, TTL and
CMOS.
Indicators: 6 front
panel multicolor LEDs.
Power
LED: Green, Power on; Off, no power.
USB
LED: Red, receiving USB communications; Green, transmitting USB
responses; Yellow – if a mix of transmitting and receiving.
Activity
LEDs, located next to the front panel inputs. Green, counting is
enabled and counts are occurring; Off, counting is enabled and no
counts are occurring, Red, counting is stopped and there is no input
activity; Red Flashing, paused (Running but Paused) and there is
activity at input; Yellow Flashing, Paused (Running but Paused) and
there is no activity at input. Yellow, counting is stopped but there
are threshold crossing at input.
Out
LED: Green, on, otherwise OFF.