1. Ken Kelly and Bob Conyne
Understanding SCXI™
Tuesday Aug 25, 12:45 - 2:15 p.m.
and 2:30 - 4:00 p.m.
Lavaca (6B)
Ken Kelly and Bob Conyne
SCXI R&D
Welcome to Understanding SCXI! SCXI stands for Signal Conditioning eXtensions for Instrumentation.
This presentation provides a more thorough understanding of the hardware and software architectures used to implement SCXI. This more in-depth look provides users the ability to quickly understand and trouble shoot any issues that may arise when assembling and programming an SCXI system.
With this extended knowledge of SCXI we hope you are able to use SCXI to its maximum potential.
Understanding SCXI™

2. Outline General SCXI background SCXI hardware Ken Kelly
SCXI software Bob Conyne
DMM & switches Demonstration with Scott Ziffra
Question and Answer
The presentation consists of a brief background description of SCXI and how it fits into the data acquisition (DAQ) system.
Ken Kelly presents the first section of the presentation on hardware specific issues that deal with SCXI communication, SCXI backplane architecture, scanning issues, and more.
Bob Conyne presents the second section which deals with the software aspects of SCXI from a NI-DAQ™ architecture and programming standpoint.
Scott Ziffra demonstrates the latest NI development in SCXI and DMM technology.
There is a 15 minute period at the end of the presentation for any questions that you may have.
Understanding SCXI™

3. Signal Conditioning Signal conditioning Multiplexing Amplification
Isolation
Filtering
Excitation
Cold-junction compensation
Signal and device switching
Analog outputs
Digital I/O and relays
Transducers and signals
Thermocouples
RTDs
Thermistor
Strain gauges
Voltage sources (mV,V)
Process current sources 4 to 20 mA, 0 to 20 mA
DAQ devices are extremely precise and somewhat delicate pieces of instrumentation. Analog inputs are within +/- 10 volts, the digital lines are meant for TTL types of signals, and the power available from the analog output is limited. This device, by itself, provides to the user a very accurate and flexible system; however, when placed in the environment of a manufacturing floor or the extreme conditions of a research lab, the signals need to be conditioned before being sent to the DAQ device.
The signals are typically varied in their behavior depending on the physical phenomena being measured. Some examples of these signals are temperature, strain, and power line signals. The transducers used to measure these signals will be things like thermocouples, strain gauges, light sensors, accelerometers, and high power relays.
Some SCXI modules and their functions are briefly described below:
SCXI-1100, SCXI Channel Analog mux modules with gain and filters
SCXI-1120, SCXI-1121 ISOLATED Analog input modules
SCXI channel sample and hold module
SCXI programmatic cutoff elliptic filter module
SCXI-1160, SCXI-1161 Mechanical relay modules
SCXI-1162, SCXI-1163 Digital input/output module
Understanding SCXI™

4. SCXI System
An SCXI system consists of PC-based data acquisition, an SCXI chassis filled with modules of various types, transducers connected to the modules through terminal blocks, and the sources of the signals.
SCXI was originally considered more of a peripheral to data acquisition than an integral part of the complete data acquisition system. This viewpoint has since become that of considering SCXI to be a necessary component of a total and complete data acquisition system for all areas of industry.
Understanding SCXI™

5. SCXI Communication SCXI is a chassis-based system
There is no computer bus interfacing
Communication is through the DAQ device
The digital I/O lines become the bus
All analog input signals come back to the DAQ device
The SCXI system specification is based on a simple three-wire serial communication interface. The communication specification is abstract, but is geared to work specifically with National Instruments data acquisition (DAQ) devices. The communication lines when using a DAQ device are implemented by using some of the digital I/O lines available at the front connector of a DAQ device. This is a small sacrifice in DAQ device functionality to get the tremendous increase in capability and flexibility that SCXI affords you.
Another important aspect of SCXI analog input modules is that no digitizing of analog signals actually takes place within the module. The analog signal after it has been conditioned is passed through a shielded cable to be digitized by the DAQ device.
Understanding SCXI™

6. Digital Communication Lines
ExtStrobe - Communication clock
ScanClk - Channel advance clock
Serdatin - Data from DAQ to SCXI
Serdatout - Data from SCXI to DAQ
D*/A - Data or address register
INTR - SLOT 0 select
These are the minimum lines required to allow for communication between the DAQ device and the SCXI chassis.
These lines correspond to certain digital and control lines of the DAQ device. These lines must not be used for any other purpose or communication with the SCXI chassis may fail. For instance, if your are using an SCXI-1180 to break out all of the DAQ device lines do not connect anything to these specific lines.
NOTE:
The ExtStrobe and ScanClk lines are common to all DAQ devices; however, the are used solely by NI-DAQ™ for SCXI communication purposes. You as a programmer using NI-DAQ cannot control these lines.
Understanding SCXI™

7. Digital Line Resource Map
Each family of DAQ devices dedicates specific digital lines to perform the communication with the SCXI chassis. This allows you to see which lines get used up by the SCXI system and these are the lines that you want to avoid using for your specific needs.
Understanding SCXI™

8. SCXI Slot 0 and Digital Communication
Slot 0 contains a Slot select register
Each module slot receives its own Slot 0 select line from this register
The Slot select register determines which module is “Awake” for digital communication
Slot 0 of an SCXI chassis refers to the power supply portion of the chassis. Slot 0 contains more than just a power supply. It also contains circuitry to control and organize module communication.
The slot select register is accessed when the Slot0Select line is asserted. This tells all of the modules to ignore any incoming data. This allows the DAQ device to write to the slot select register. Once a module number has been written and the Slot0Select line has been deasserted all further communication will be between the DAQ device and that specific module.
When a different module is used, the process simply repeats.
Understanding SCXI™

9. Class I / Class II Class I modules Class II modules
One data register (D*)
Module ID register (A)
D*/A acts as a register select line
Class II modules
Multi-register architecture (EEPROM)
Must write the address and then the data
Module ID register on every module
This ID uniquely identifies each module type
There are two register decoding schemes in the SCXI specification:
Class I modules are the simplest modules to communicate with. They contain only one data register and the module ID register. Some of the modules that follow this architecture are the SCXI-1120 and SCXI The D*/A line becomes a register select line in this architecture.
Class II modules allow for an expanded number of registers to be accessed. Any module that has additional software configuration will require it to be a class II module. For instance, a gain or configuration register or the ability to read a EEPROM. Examples of this are the SCXI-1102 and SCXI-1126.
Understanding SCXI™

10. SCXI Scanning Slot 0 contains a channel count FIFO
Module contains the start channel
Slot 0 selects which module accepts the ScanClk by using the SCANCON line
The module resets to its start channel when it is deselected.
Slot 0 controls the scanning operation by maintaining a FIFO that is filled with the number of pulses from the ScanClk line that each module gets. If the module in slot 1 is to scan 5 channels and the module in slot 2 is to scan 10 channels, then the slot 0 circuitry will select module 1 and when it has received 5 pulses from the DAQ device it deselects module 1 and selects module 2 after module 2 has received its 10 pulses then it is deselected and module 1 is selected. and the process repeats if so programmed.
You can think of the SCANCON line in this fashion:
The SCANCON line is to module scanning as the SlotSelect line is to digital communication.
When a specific module has its SCANCON line deasserted it resets itself to be connected to its programmed start channel.
Understanding SCXI™

11. Analog Bus Routing SCXI Chassis SLOT 0 DAQ Channel 0 DAQ Board
MUX &
Conditioning
Circuitry
DAQ
Board
DIO Lines
Input
Signals
MUX &
Conditioning
Circuitry
This is for scanning
module number 2
ANALOG BUS
This slide shows how the analog bus is switched in and out of a module’s analog path. Each analog input module has the ability to connect its own signals to the analog bus. If a particular module is the cabled module, then it will utilize the circuitry it has to connect either the analog bus or its own signals to channel 0 of the DAQ device.
This behavior is dependent on the assertion and deassertion of the SCANCON line.
MUX &
Conditioning
Circuitry
Understanding SCXI™

12. Cabling DAQ Board to SCXI
In general it does NOT matter
There are a couple of exceptions
SCXI-1140
Uses an additional digital line from the DAQ device
Digital Modules
No analog bus support
Parallel or Multiplexed operation
One particular module requires an extra line off of the DAQ device. This line is used by the SCXI-1140 to perform simultaneous sample and hold. Because no other module currently uses this line, if you have an SCXI-1140 in your system you must cable it to the DAQ device.
Digital modules do not access the analog bus at all. This means that you cannot read an analog module if you are cabled to a digital module.
Remember that you can access a digital module while acquiring data from an analog module. This is possible because once the chassis and analog modules are programmed with a scan list, the digital lines are no longer required through the entire acquisition process.
Understanding SCXI™

13. Parallel versus Multiplexed Mode
Parallel mode
Limited to <8 channel analog modules
Analog lines always energized
Multiplexed mode
Uses the analog bus and slot 0
The analog bus is switched by the modules to route the signal to the DAQ board
There is a small set of applications that may require the use of parallel mode.
There are several things to be aware of should you choose this option:
1) The DAQ device can only access the signals that the module it is cabled to provides.
2) You no longer refer to SCXI in your programming environment, you are simply reading the DAQ device channels.
3) There is a module cascading kit that can allow one module to feed its rear signal connector signals to the front of an adjacent module.
You can operate 32 channel digital modules in parallel mode; however, you must use a digital DAQ device, such as a DIO-32HS.
Understanding SCXI™

14. Parallel Mode
This diagram shows that in parallel mode the module ignores all backplane communications and passes all conditioned analog signals directly through to the rear-signal connector.
Understanding SCXI™

15. Multiplexed Mode
When a module is configured in multiplexed mode it utilizes all of its switches and communication lines to successfully coexist with other modules in the SCXI chassis.
Understanding SCXI™

16. Quiz The SCXI-1124 is an analog output module
Question 1: Does this module use the DAQ board analog output?
Question 2: Is the SCXI-1124 an analog module or a digital module?
The SCXI-1124 does not use any analog output resources of the DAQ device. In fact, the SCXI-1124 is purely digital module. The use of digital to analog converters on the module makes this possible.
Understanding SCXI™

17. Terminal Blocks Connectivity options Cold-junction compensation
AC/DC coupling
Attenuation
BNC (BNC-2095)
Thermocouple (TC-2095)
TBX series
Cable away from module to terminal block
Terminal blocks are the final component of an SCXI system.
Terminal blocks provide the following features:
Isothermal – For consistent thermocouple (TC) reading across all channels
Cold Junction Compensation– Required for TC readings
Attenuation– Needed for measuring high voltage signals
AC/DC coupling– Needed if an AC signal is superimposed on a high common mode voltage
Terminal blocks also provide connectivity options such as screw terminals, BNC connectors, and thermocouple spade connectors. The TBX series includes a cable that allows you to make your field connections away from the chassis.
Understanding SCXI™

18. PXI 1010 Chassis Signal conditioning for PXI™
Internal bus (no external cabling)
There must be an MIO device in the PXI slot that is right next to the SCXI portion
Additional SCXI chassis may be attached
SCXI and PXI™ come together. A PXI-1010 chassis with SCXI connected to LAN becomes a remote system through the use of RDA. This allows for high throughput and connectivity only limited by your LAN.
There is an internal local bus that connects a dedicated channel of the PXI-MIO board to the analog bus of the SCXI system next to it. This eliminates the need to cable to the rear-signal connector of a module in the SCXI sub system.
Understanding SCXI™

19. Outline SCXI gain considerations New SCXI switch module
What’s coming for SCXI
Topics to be covered during this section:
SCXI gain considerations – how LabVIEW™ string syntax affects gain calculations, and how to use AI Parameter to determine your SCXI gain.
SCXI switch module – an exciting new product that expands the functionality of instrument-class DAQ products like DMMs. New capabilities include matrix/independent switching, random scanning, and new LabVIEW string syntax.
What’s next – a look at future directions in SCXI, including SCXI architectural changes, a new C API, OEM support, and a demo of the new autodetection capability in our configuration utility.
Understanding SCXI™

20. Gain Considerations LabVIEW™ string syntax implications
Using AI Parameter to determine gains
Two areas are examined for SCXI gain considerations:
The affect of OB0! in LabVIEW strings
The use of AI Parameter to determine the SCXI gain
Understanding SCXI™

21. SCXI Strings in LabVIEW: A Review
OB0!
SC1!
MD2!
CH2:7 .
Understanding SCXI™

22. SCXI Strings in LabVIEW: A Review
OB0!
Onboard channel 0,
the DAQ Board AI
channel that
corresponds to
the SCXI chassis
SC1!
SCXI Chassis 1
MD2!
SCXI Module 2
CH2:7
SCXI Channels 2 - 7
(‘CH’ is optional)
OB0! – onboard channel 0, the AI channel that the DAQ board uses to digitize the input signal coming from the SCXI chassis. One channel is assigned for each SCXI chassis you have cabled.
SC1! – SCXI chassis 1, used to distinguish between chassis in a multi-chassis system (but required even for a single chassis system).
MD2! – module 2, to refer to the slot number of the module you are programming.
CH2:7 – the channel list for scanning. Note that the ‘CH’ is optional.
Understanding SCXI™

23. LabVIEW String Syntax Board: PCI-6031E Module: SCXI-1102
Input range: to 0.01
Board gain= 5
OB0!SC1!MD1!0:7
Module gain=
100
(board input range = +/- 5V)
For a given board/module combination, using OB0! instructs LabVIEW to try to optimize the gain settings to get the largest possible gain. Omitting OB0! means that the board gain will be fixed at 1.
Board gain= 1
SC1!MD1!0:7
(board input range = +/- 1V)
Module gain=
100
Understanding SCXI™

24. Conclusion: if you need gain, always use OB0!
In most cases, using OB0! in LabVIEW channel strings makes the most sense.
Understanding SCXI™

25. Using AI Parameter to Determine SCXI Gain Settings
AI Parameter: A Generalized Get/Set Attribute VI
parameter name
float out
operation
parameter name
operation
float out
AI Parameter is a generalized Get/Set Attribute VI for analog input operations. It allows more granular control of the AI capabilities of DAQ hardware devices, including SCXI.
Understanding SCXI™

26. Using AI Parameter to Determine SCXI Gain Settings (Continued)
Operation options:
0: Set
1: Get
2: Translate
Parameter name options:
0: Notch filter frequency
1: Measurement mode
2: Open t/c detection
3: Ground referencing :
13: SCXI Gain
To determine your SCXI gain in a LabVIEW program, you want to GET the SCXI GAIN, so these are the settings we illustrate.
Understanding SCXI™

27. Using AI Parameter to Determine SCXI Gain Settings (Continued)
task ID in
task ID out
error in
error out
The above slide provides an example of how to determine the SCXI gain in a LabVIEW program.
Understanding SCXI™

28. SCXI-1127 Switch Module Switch modes – independent, matrix
Random scanning
New LabVIEW string syntax
The SCXI-1127 is a relay multiplexer that, along with a new high-voltage backplane connector for the SCXI chassis, can be used for complex signal routing to provide sophisticated switching for MIOs, DMMs, and other instrumentation-class devices. The module can operate in independent or matrix mode (matrix mode is initially supported in NI-Switch only), and supports random scanning via a new channel string syntax in LabVIEW.
Understanding SCXI™

29. Switch Modes Matrix: uses row, col syntax
Independent: open/close switch, similar to existing digital I/O products
Matrix mode allows the routing of any channel to any other, so for example, you might route a DMM input to a device under test.
Independent mode allows opening/closing of individual switches and is similar in operation to existing digital relay modules like the SCXI-1160.
Understanding SCXI™

30. Sequential Scanning num chan mod # start chan start chan start chan 12 7 5 2 4
OB0!SC1!MD1!CH7:8
OB0!SC1!MD2!0:3
OB0!SC1!MD3!CH5:7 3 3
Mod 1
Mod 2
Mod 3
Slot 0
The current SCXI scanning model requires that SCXI channels be ascending and contiguous. This is due to the hardware limitation of a single “start channel” register per module. Slot 0 has a FIFO to store the number of channels to scan for each module participating in the scan.
Understanding SCXI™

31. Random Scanning chanlist chanlist num chan mod # chanlist 1 2 7 5 6 24 9 2 6 3 3 7 1 13
Slot 0
Mod 1
Mod 2
Mod 3
Beginning with the SCXI-1127, all future modules support random scanning. To accomplish this, SCXI modules have a FIFO to store each individual channel in the scanlist.
Understanding SCXI™

32. New LabVIEW String Syntax for Random Scanning
Current syntax (still supported): OB0!SC1!MD4!0:11
New random-scanning syntax: OB0!SC1!MD4!(3,0,5,11,5)
The existing LabVIEW channel syntax is still supported, but the additional capability of creating a random scanlist has been added. Notice that channels need not be ascending, and can even be repeated in a scanlist.
Understanding SCXI™

33. SCXI Future Directions
Complete SCXI rewrite in progress
OEM support
New C API
Autodetection!!
SCXI rewrite – to make SCXI leaner, meaner, easier to maintain, and easier to add support for new modules.
OEM support – an area we are looking at very seriously is how to make SCXI a more open standard. The goal is to make SCXI easier to integrate into OEM applications.
New C API – C code that performs an AI operation is virtually the same, whether the AI channels are on a DAQ board or an SCXI module. Using a channel string to initialize a session, all C AI functions will then use this session ID to perform I/O, similar to the LabVIEW model.
Understanding SCXI™

34. Autodetection Demonstration
A demonstration of our exciting new autodetection capability in the NI-DAQ™ configuration utility.
Understanding SCXI™