The CNA is a precision control and monitoring module that is designed to be used as part of a Group3 Control system. It is functionally equivalent to a type A board and Processor and the signal conditioners all rolled into one unit.
The CNA can also implement closed loop control, as it has PID algorithm built in to it.
It is housed in a low profile metal case, and is designed as a very rugged, small, inexpensive unit to aid the implementation of a distributed control system.
The unit consists of a main circuit board contained entirely within a metal enclosure, linked by a short length of 40way ribbon cable to an I/O board (SCA) mounted on top of the case.
Two Analog Input channels
16 bit resolution, bipolar, each with differential inputs.
Both channels sampled 30 times per second.
Channel 0 can be used as the feedback input, if PID control is selected.
One Analog Output channel
16 bit resolution
Output signal selectable as:
Output Impedence 100W
The analog output acts as the closed loop PID control output, if selected.
Eight Digital Inputs
Opto-couplers on each channel can be selected to operate in two modes:
1. Signal powered channels are completely independent, require minimum of 5mA signal.
2. Contact closure channels share a common return potential (which is isolated from the rest of the unit up to 500V).
Eight Digital Outputs
Communications by fibre optic cables at 1.15Mbaud. Entire unit is isolated from the controlling computer.
Power supply input required:
18 to 36 volt DC, or 14 to 26 volt (rms) AC, 3 Watts.
Unit contains an internal switch mode supply, providing isolation up to 200 volts.
Fully featured Diagnostic Port, allowing configuration and local control over-ride from a terminal with an RS-232 serial port.
Two stage transient suppression
All I/O pins on the main board have fast acting, voltage limiting components installed, coupled with further suppression and isolation on the I/O board.
Guidelines for determining the hardware required for an installation.
The number and type of Device Interface units (DIs) required is determined by the number of I/O channels, the physical layout of the units, and the different electrical potentials in the system.
All channels within a DI case must be at the same potential.
This is all channels on all cards in the same DI. Give some thought not just to normal running conditions, but also to what happens on a breakdown. If there is a chance that, say, one power supply could drift from the potential of another supply on a breakdown, then the two supplies should be controlled and monitored using a separate DI for each.
To take advantage of the noise immunity offered by the fiber optic cable, the DIs should be placed as close as possible to the device they are controlling. All copper wires bringing signals to the DI should be as short as possible - preferably less than half a metre. Place the DI right at the back of the power supply, switch panel etc. This means that in a noisy environment two supplies more than a metre or so apart , even if they are at the same potential, should really be controlled using a separate DI for each. This wiring length restriction can be relaxed if the DI and supplies are inside a shielded rack enclosure. Digital signals can be isolated using relays or optocouplers (available on Group3 signal conditioners) to overcome these distance limitations. This will permit, for example, all the signals from the physically distributed door safety switches to be sensed from one type B digital board.
Bearing in mind the two important factors above, it is necessary to determine the I/O cards needed for the DIs, and which size DI to use at each point. DIs are available that accept one or three I/O cards. A DI does not have to be fully populated to function, so a slot could be left empty to allow for future expansion at that point. A DI can be populated with any combination of I/O boards - all different or all the same. A single CNA module offers the smallest way to control and monitor a typical power supply, providing two analog inputs, one analog output, 8 digital Inputs & 8 digital outputs.
A Step by Step Guide to determining hardware requirements.
1. Make a list of all the equipment to be controlled and monitored.
List each power supply, interlock, current monitor, temperature sensor etc.
2. For each piece of equipment determine the analog and digital signals needed for control and monitoring. For analog signals, establish the range and polarity of signal present.
For example a listing for a typical power supply may have:
So to institute full control and monitoring of this power supply would require a total of 1 analog output, 2 analog inputs and 3 digital channels from a Group3 DI.
List every control or readback channel in the system, with the sort of detail as shown in the example above.
This takes some thought and considerable time on a big system, but it has to be done at some point if instituting computer control.
These channel by channel lists are also essential when configuring a control software package, so a neat orderly listing defining all the channels at the start of a project saves time in the long run.
3. Separate the channel listings into voltage potential levels.
Eg. separate into one section all the channels that are up at the ion source potential, into another all the channels at ground potential etc.
All the channels being controlled or monitored from a particular DI must be at the same potential, and any potential difference has to be traversed using fiber optic cables to another DI. So there are times when only a few out of the eight analog channels on an I/O card can be used. For example if the Ion source level only requires 4 analog inputs to monitor it, then the remaining 4 analog inputs cannot be used to measure any signal at other potentials.
4. Further subdivide the lists at each potential level into channels that are in the same physical location.
In an electrically noisy environment the signal wires should be kept as short as possible to minimise transient pick-up. Part of the system’s reliability comes from keeping signal wiring short and using the fiber optic cable to traverse any distance. Ideally signal wiring should not be more than about half a meter long. This means that equipment within a rack cabinet could be controlled from one DI, but that equipment in a separate rack cabinet a few meters away should be controlled by another DI.
5. Determine the I/O boards and DIs required.
There should now be a series of lists of channels, separated according to voltage potentials, and further subdivided according to physical location.
On a small system all of the channels at the same potential and at the same location could be controlled or monitored from a single DI, but usually the channels have to be assigned to a number of I/O boards , which are then housed in the appropriate number of DIs.
Divide the number of each type of channel in the subsystem by the number of channels on that type of I/O board (see list below). Rounded up to the nearest integer this gives the number of each type of I/O board required for that sub-system.
Number of channels per I/O board
2 analog in, 1 analog out, 8 digital
24 digital channels
8 analog inputs
8 analog output
4 DC motors
2 serial comms ports
4 stepper motors
2 precision analog out
1 GPIB / IEEE 488 port
These I/O boards will then need to be housed in DIs. There are two sizes of DIs available - housing one or three I/O boards. A DI does not need to be filled to capacity, so unless there are severe space constraints it is sensible to standardise on three slot DIs. This also allows for future expansion at that point.
A DI can be filled with any combination of up to three I/O boards. They can be all the same, or all different. The only exception is that there can only be one Type F or one Type K board in each DI. If several DIs are needed at one point then it doesnt really matter which I/O boards go in which DI.
So at this point it should be possible to draw up a list of the DIs, filled with the required I/O boards. eg.
6. Loop Controller(s)
The Group3 loop controllers handle the communications on the fiber optic loop, allowing the control computer to talk to the DIs.
A single loop can have up to 16 DIs operating on it.
There are some points that need to be considered before deciding on the number of loops an installation requires:
On average the response time of a particular channel on a large system is 0.8 ms per I/O board on the loop. 16 DIs can contain up to 48 I/O boards, so the response time on a fully loaded loop can be nearly 40ms. If this is not acceptable then the heavily loaded loop should be split into two smaller loops, each requiring a loop controller.
Communications on the loop will be broken if a DI does not have power supplied to it. If power can routinely be disconnected from some DIs, but control is still wanted from channels in other DIs then these two sets of DIs should be controlled on separate loops.
There is no inherent limit to the number of loop controllers that can be installed in one computer. The hardware design of the loop controller makes each loop appear to the control computer as a small (2 Kbyte) section of memory. All the communication is handled by the processor on the loop controller so adding more loop controllers into a computer does not slow down the computer hardware. The only limit to the number loops that can be run from a single computer is the number of expansion slots available in its chassis - but even then another computer can be added to the first on a network.
Deciding which type of loop controller will depend on the hardware your institution has, or intends to use.
If designing a new system from scratch it is probably most cost effective to choose to use a PC. They are inexpensive, widely available, and can operate with a number of very useable software packages.
Loop controllers currently available are:
The prime function of the Group3 signal conditioner range is transient absorption.
They are highly recommended in any electrically noisy environment.
They also ease the task of installation wiring by providing rows of screw terminals to attach the signal wiring to.
Signal conditioners available are:
8. Fibre optic cables
The standard fibre optic cable used is Hewlett Packard simplex plastic cored cable.
The maximum length of any one piece of plastic cable for use with Group3 Control is 40 meters. Each DI receives, decodes and re transmits the loop data, so every cable in the system could be 40 meters.
Alternatively, a new HCS, silica cored fiber that fits the plastic connectors is available. Maximum distance with this new cable is 500 meters in any one length.
Another option is to order the hardware with glass fiber optic cable transmitters and receivers. The hardware supports ST (bayonet type) or SMA (screw type) connectors. Using glass cored cable distances of up to 3000 meters can be covered with one cable.
9. Power supplies
The DIs require a local supply of between 12 and 26 volts, DC or AC.
A domestic wall plug power supply of a suitable voltage can be used provided it has a rating of more than 10VA.
Group3 can supply a small power supply that mounts on a DIN rail capable of powering 1 or 2 DIs. This unit, called a PS24D15, provides an isolated 24 volt DC, 15W output from a universal mains AC input (100 to 240 volt AC). The isolation is particularly valuable in a noisy installation to reduce mains-borne transients.
10. Diagnostic Port cable
To allow a standard terminal to talk to the DI diagnostic port.
The pin-out of the connector on the DI is given in the users manual, or a ready built cable is available from Group3, part number DPC2.
Following the preceding points 1 to 10 should generate a list of the DIs, with the I/O boards, the loop controller(s), signal conditioners and accessories - in fact the complete list of Group3 hardware required to complete an installation.