|
550 539 02 |
||
|
|
||
|
New Features in the PD 600 Series of modulesThe PD 600 Series of modules takes advantage of certain facilities offered by the P-NET fieldbus protocol, to enhance ways of transferring data between P-NET nodes. These features are discussed below, and include:
NotificationThis feature is built into all 600 series I/O modules, and will be recognised by the inclusion of the previously unused Register 2 in all Service channels having the identifier Notification. The purpose of this register, which is structured as an array of 8 individual bits, is to indicate whether there has been a change in value of certain internal variables. Each bit is associated with a separate I/O channel and also one with the common actual error register in the service channel (ChError.ComAct). The operation and meaning of each bit is determined during configuration of each I/O channel (ChConfig), where an active bit could, for example, indicate the rising or falling edge of a digital input, or a change in the level of an analogue input. Having just a single byte to poll by a master device, to establish whether any change has occurred within a particular module, will minimise traffic between a master and slave due to a reduction in individual polling. If a change is detected, such an event can be used by a program in a DPI, PC or other controller to generate some kind of interrupt.
Super Global VariablesThe standard way of sending a message via P-NET is that a master device (DPI, PC, Controller) makes a request to a slave device to store (write to) or load (read) the value of a variable identified by a particular softwire number (SwNo). Although all devices receive this message, only the device that has the same Node Address (NA) as included in the message will perform the requested action and respond with a confirmation message.
The idea behind the use of Super Global Variables is that nodes often hold variables whose value needs to be synchronised with another variable, such as with a timer or some measurement value.. A variable whose value can be changed and/or distributed in this way is termed a Super Global Variable (SGV), and is identified in a project as having a unique name. The way that the value of an SGV is distributed, is achieved by the requesting master using a particular node address that is recognised by all nodes (NA = 126). This is known as a broadcast node address. However, only those slave nodes that have been designed to deal with Super Global Variables will process this received message any further.
All 600 series I/O modules utilise a newly defined configurable register in the Service channel called SGVTable. As the name implies, this is structured as a table where the SwNos of specific SGVs are entered. Against each of these is entered the SwNo of an internal variable of the same data type, together with whether this module is to be defined as the exclusive “owner” of the SGV. When a message is received, the modules test to see whether the SwNo matches an entry in the table. If there is a comparison, and the request is to store (write), then the value is written to the associated internal SwNo. The module that has been defined as being the owner of the SGV, then generates a normal response to the request message. However, if the request from the master is to load (read), then the owner of the matched SGV will read the current value of the associated internal SwNo and produce a further broadcast response (NA = 126) to store this value in any node having the same SGV entered in its SGV table.
This mechanism provides an efficient means to change the value of a variable in many modules (Super Global variable), or to transfer the current value of the owned SGV to other selected modules. This means that activities such as closing down a number of digital outputs at the same time can be achieved at a higher speed than addressing each individually. It also enables the distribution of say an analogue measurement or set point from one slave to other slaves, or the synchronisation of timers or clocks over a number of nodes, to all be achieved within the period normally required for just one confirmed P-NET request.
The concurrent use of the Notification and Super Global Variables features provides enhanced facilities for distributing time critical data around a network at high speed. For example, the polling of an input signal for a change of state in one module, could stimulate the closure of individual valves in many other I/O modules, within the period required for just two P-NET transmissions. With this example however, it should be noted that since there is no confirmation feedback when using such a strategy on its own, there is no guarantee that outputs will have actually changed state.
Cycle Time ReductionThe “standard” communication rate of P-NET (76800 bps) has been carefully chosen as an ideal practical compromise between speed (3mS cycle time) and distance (1.2 Km) using RS485 without repeaters. The use of the Light-Link optical medium for transferring data the short distance between 600 series modules within a cluster, provides the possibility of increasing the speed of data transfer. It is now possible to select a speed of 230400 bps, which is three times the rate used for RS485 connections. (see Baudrate in DPI and NetParam in I/O module). Hot SwapSeries 600 modules have been designed so that individual electronics units can be changed without effecting the operation of others within the cluster.
Object VisualisationWhen using PC HMI and SCADA applications, such as Visual VIGO, the new features described above provide a certain enabling functionality towards the enhanced visual use of Object Oriented Programming and the use of Object Function Blocks. However, since such visualisation applications can be regarded as the user interface to a master node (PC), the use of Super Global Variables as Variable links in Component Objects will also reap the advantages of distributing a value to many PC applications at the same time. The fact that receipt of a new value using this facility makes it possible to reset the cyclic timer (VIGO Timer), which defines the period for automatically obtaining a new value, will also help to reduce traffic on the network. This is especially useful when a number of PCs are running the same application such as Visual VIGO.
Data AcquisitionVisual VIGO has the ability to log and display actual and historical data, but it is necessary for the PC to be permanently connected to ensure that all values are stored. 600 series DPIs are now capable of incorporating a group of DataCollect Channels, which will enable logged data to be stored locally. This means that PCs need not always be connected or powered, and allows portable PCs to extract and display logged data at any time. The operational use and configuration of DataCollect Channels in DPIs is a function provided by specific Data Logging components provided for use with Visual VIGO. The availability and current functionality of such components can be found by referring to the Visual VIGO General Help manual or the Visual VIGO Components and Component Elements manual. Contact PROCES-DATA for further information.
Related topics
|
|