TechArchive

Original article date: April 1998

Have Micro PLCs reached their limit? Probably in terms of size, say leading manufacturers, but many other issues are still developing fast. Alan Quinn rounds up the latest opinions.

Micro PLCs may have now reduced in size to an extent where they are probably as small as they can be whilst remaining practical. In most cases, the PLC will have wires connected to the I/O and this requires screw terminals or connectors. That’s the view of Ian Purcell form Matsushita, whose FP0 measures 90 x 60 x 25 mm for a 32I/O CPU and is one of the smallest. Keyence’s Simon Smith agrees, while expecting further processing power and functionality within Micro PLCs of the same size.

There seems to be little agreement on what constitutes the best definition of a micro PLC and company views are perhaps more blurred than a year ago. Rockwell no longer splits the range cleanly at 128I/O According to Product Manager Richard Sturt, the Micrologix range goes to 30I/O, but then the SLC takes over and goes to several hundreds of I/O. Omron’s Paul Downey classifies micro controllers as those being able to address up to 90 to 100 I/O. As this cross-over point is approached, selection of suitable controllers tends to be as much based on functionality than on pure I/O capacity and therefore it is as likely that a mini controller would be used as a micro controller. On the other hand, the Mitsubishi view is that the division should perhaps be at 256 I/O.

Generally, faster CPUs and more powerful instruction sets are seen to be the future. Ian Purcell points out that PID functions, floating point maths, stepper motor control, PWM control and interrupt processing are all now available in a Micro PLC. He expects the next few years to see the incorporation of built-in communications features and networking standards like ASI and Profibus. Built-in analogue and model communications are likely

Although several new products have been introduced by Rockwell in recent times, Product Manager Richard Sturt does not see any dramatic developments in the future. Trends are towards to smaller units with lower cost, more distributed architecture and networking capabilities.

Chris Hazelwood (Mitsubishi) believes that new markets are being opened up by intelligent product design and the implementation of new technology. It is hard to say the Micro PLC of today bears any resemblance to the PLC of yesteryear and with more power and technology being poured in to the Micro PLC of today, tomorrow’s PLC will be different again.

Micro PLCs are pushing the boundaries of control in two directions, up to meet the small systems user and downwards in direct competition with many “in house” dedicated board solutions. Chris Hazlewood says that Mitsubishi and other leading manufacturers are acutely aware of this and try their very best to seek a middle ground. Mitsubishi is addressing the dedicated controller market by offering products such as FX0S, which has high speed counters (up to 7kHz). The latest micro PLC product, the FX2N, is designed to push the Micro PLC envelope even further, with the inclusion of auto tuning PID, and processing speeds comparable with the highest process controllers, large program memories and data storage facilities and a built-in RTC functions. Taken with a communications explosion, it is very hard not to see a micro PLCs successfully bridging the gap that was traditionally found between standalone micros and system rack PLCs.

In general terms Chris Hazlewood sees control and automation moving towards the exploitation of technologies such as Windows CE, JAVA /JAVA Virtual Machines and would not be at all surprised to see Ethernet as the preferred network even down at sensor level. Why? Because these technologies are faster developing due to the very nature of the personal computer business. Investments in company infrastructures and business operations will dictate the rationalisation of the networking and to some extent the programming methods. On more and more occasions the company IT Manager is being called in to provide input on what the communication and operational strategies of the company business should be. Even so, local, standalone control with the ability to integrate to a higher network will always be a requirement.

Paul Downey points out for Omron that micro controllers are being used in places where previously there may have been standard I/O driven by a larger controller. The micro therefore offers localised intelligence in addition to I/O, enabled by networking facilities within micros. This is reinforced by Simon Smith who says that no longer is an entire machine controlled by one huge PLC. Now, it is sectioned with smaller parts controlled by micro PLCs. Each one of these can then be linked to another micro PLC, or a controlling software package which coordinates the machine in its entirety. And what about retrofits? As devices are retrofitted to machines, they will need some control system. If the current PLC does not have enough space, then a micro PLC can be added to operate the new portion.

New Products

Available in four sizes, Omron’s CMP series is aimed at applications form 10 to 100 I/O. It incorporates interrupt processing, input filters, high speed counting, built-in analogue potentiometers and flash memory. RS232 and RS422 interfaces are included. The Syswin programming software, as discussed last year, is compliant with IEC1131-3, and is the same package used to program every other PLC in the Omron range.

A brand new feature of the Micrologix is embedded analogue. Richard Sturt says that most manufacturers offer this as a separate module, but Rockwell is just introducing three different versions with a choice of power supplies and input voltages. The controllers have 12 digital inputs, 8 digital outputs, 4 analogue inputs and a single analogue output.

For networking, there is now a Rockwell module that allows users to have highly distributed controllers, integrated on a network, running under DeviceNet.

Matsushita’s multi-function analogue unit is available with two channel in and one channel out. The unit can utilise various voltage, current and thermocouple input and output ranges.

Also new is a modem capability with the Micrologix down to 10I/O. The DFI half-duplex is aimed at SCADA applications or remote diagnostics. Matsushita also offers a modem function that allows the CPU to alternative between a master and slave environment.

One of the new developments from Keyence is the recently released KV ladder builder for Windows software. The package contains all the original features of the DOS version. It improves the time taken to debug a PLC program by allowing the program to be monitored as it runs, without needing to first download it to the PLC.

Keyence has also launched analogue-to-digital and digital-to-analogue extension units for the KV series. Supplying four-way A/D or D/A conversion, these new modules need no extra software and simply plug into the side of any of the KV range.

First Time Users

According to Omron’s Paul Downey, software for programming and support of the first time user must be intuitive. But having gained experience with these devices, subsequent applications may well require more I/O, different functionality or other networking options. This makes it important to provide synergy throughout all the controllers in the range. Omron offers an introductory application guide to accompany its new CPM series, which covers hardware, software and basic applications.. There is also an extensive training programme managed by a full-time training team.

For first time users, there is a new offering from Rockwell. The Micrologix 1000 starter pack with RSLogix 500 software costs (UK pounds)199, including cable and input simulator. Programming under Windows 95 was introduced over the past 12-18 months and this makes it much easier for first time users.

Matsushita has recently introduced the FPSOFT Windows programming software, which can be used to program the full range of Matsushita PLCs. Again, the FP0 starter kit has been introduced this year, geared towards the user of relays, wiring, timers and counters. The inclusive course aims to bring a new user up to a basic level within hours.

Mitsubishi sees two main areas where first time users, not only of Micro PLCs but of all automation products, need additional support and guidance; initial product selection and product commissioning/programming. Mitsubishi helps users in these areas by providing the following tools:

• FX-TAP, an electronic selection and system design guide for Micro PLC users
• FX-PCS/WIN Windows-based programming tool
• MX Soft MMI configuration tool for inverters
• various guides, manuals and programming tips to support users of all levels and all capabilities.

Year 2000

Is the year 2000 issue a problem for micro PLCs? The answer has to be: it depends on the application! If we assume that Micro PLCs tend not to have built in diagnostic features based on a time schedule and we assume the user has no program or function derived from a real time clock and calendar (RTC/RTCC), then no there is no problem. This tends to be the case in very many applications and most suppliers of PLCs appear content to leave it there. Not much advice, though, from many of them on what to do about existing applications.

And there are, says Chris Hazlewood from Mitsubishi, products with real time clock-calendar (RTCC) facilities built in and others which offer this as a product supplement. This highlights the need to identify where the RTCC function are within any given piece of micro electronics or Micro PLC. However, judging by the number of enquiries received by Keyence’s Simon Smith, there is no doubt that users take very seriously, although Chris Hazelwood says that it won’t mean the end of the world, even if, as Rockwell’s Richard Sturt suggests, the lights go out at midnight 1999!

Like most PLCs and associated software, the Keyence range is checked for millennium compliance, but the view taken is that this range is unlikely to cause any problems because no real-time clock or embedded date is contained within the processor.

Ian Purcell at Matsushita agrees and points out that the majority of applications performed by PLCs are not real-time dependent and so this issue does not generally present a problem. For applications where RTC is an issue, he says that it is up to the application engineer to ensure that application program can cope with the change effectively. If the Micro PLC offers RTC functions and the PLC programmer has utilised these, then the PLC programmer should check through the appropriate sections of code. In the case of the RTC in NAiS branded products from Matsushita, time information is confined to t wo digits for hours, minutes and seconds. The date information also uses two-digit data for the day, month and year and therefore ignores the century information. According to Purcell, this ensures that NAiS PLCs are Year 2000 compliant.

Omron’s Paul Downey notes that clock and calendar functions within PLCs are increasingly being used for applications such as error reporting, event sequencing and product tracking and this capability is increasingly either an option or a standard, even on microcontrollers. All controllers with such functionality should be fully compliant with the Year 2000 issue and all Omron controllers at least with clock and calendar functions have been tested.

Richard Sturt also takes the view that the Year 2000 issue is a problem for micros with real-time clocks, but the Rockwell Micrologix does not have one either. The SLC (in versions over 30 I/O) does, but it is Y2K compliant. He recognises, though, that there are hundreds of small controllers knocking around that do have a real-time clock and are not Year 2000 proof. If they are spread throughout a building, controlling the lights, they will be difficult to identify and fix, but if you don’t. says Richard Sturt, the lights will go out at midnight 1999!

Chris Hazlewood points out that even if a product does not offer RTCC functions directly to the user, they may still exist within the product itself. The manufacturer may use time or calendar functions to help provide maintenance information or perhaps just simple performance statistics. So it is important that customers do not feel afraid to ask the manufacturer for statements regarding the applicability and use of RTCC data in any micro electronic product. Mitsubishi has a statement available on request from any user.

Another issue is that, if the Micro PLC is connected back to a master network where some type of SCADA is used, not only to monitor/supervise but in 99% of situations actually provide the date and time stamping, then there is a very real possibility that the system administrator should look seriously at getting the SCADA package and indeed the computer verified for the extent of compliance with the issues of the year 2000.

Within the Mitsubishi FX family of Micro PLCs, the F, F1, F2, FX0 and FX0S have no RTC function at all. Customers may be confused and ask “How is it then, that your PLC can provide timer operations?”. This is simply explained: the generic timers used in all Mitsubishi FX family of PLCs are actually counters! Instead of receiving external inputs to trigger the count, the PLC actually counts pulses output from a quartz crystal. Hence, when you see a timer with 100ms duration, what you are actually seeing is a counter with a limit of 100 counts of 1ms clock pulse, or a 100,000 counts of 0.0001ms.

The Mitsubishi FX0N, and FX products also have no RTCC function built in. However, a supplementary RTC memory cassette is available. The RTC cassette offers a time period calendar function. In this particular example that is until the year 2079. It is quite a common practice to offer a specific time frame type operation – for example videos typically will only have a 10 or 15 year clock function. Because the time duration is controlled, extensive and appropriate testing allows the manufacturer to ensure the RTCC product performs correctly. In the case of the Mitsubishi FX RTC memory cassettes, they have been tested to ensure that they recognise both the change to the year 2000 and the recognition that the year 2000 is actually a leap year. So the PLC will recognise and process “time as we know it” correctly.

But as with any other piece of information supplied by the PLC, it is subject to the interpretation of the user – or specifically in the case of the PLC – the PLC programmer. Mitsubishi recommends that customers check their own programming to see that they have not created a situation where the original time/calendar data provided by the PLC can be misrepresented.

The latest Micro PLC product from Mitsubishi, the FX2N, is the first micro PLC the company has produced with the RTC function built in as standard. Mitsubishi has applied the same technology to the new FX2N RTC functions as was available in the existing FX RTC cassettes but with one important difference. To try to remove the possibilities of time/calendar data being misrepresented, Mitsubishi has added six special RTCC comparison and handling instructions, with an additional but optional function of switching the year data from two-digit to four-digit.

So all PLC suppliers have established policies for identifying which applications are likely to present a problem and to ensure that new PLCs leaving the factory gates are millennium-compliant. In conclusion the user is well advised to seek confirmation from all suppliers of equipment that are used – it only takes a few minutes or a short fax to ask the question but it may save hours of unnecessary rework or facilitate a positive action to resolve the issue.

Submissions from the following companies are gratefully ackonwledged

• Keyence
• Matsushita
• Mitsubishi
• Omron
• Rockwell

April 1998