Newsletter Issue # 15

Real Time Automation's - Best Darn Newsletter 

I Get a 300 Euro Sales Lesson
S7 Communications
Fun Facts and Trivia

Get a free RTA bag this month only!

Email your name and address to: jladd@rta- by December 28th to claim your steal of the month.

I Hate Cats
Modbus TCP vs. Modbus RTU
ASCII Revisited
My Crystal Ball Has Some Spaghetti Sauce on it, but it Still Works



Practical tips and information for young engineers. This issue, featuring:

- The Problem with Nickel-and- Diming Innovation is You Don't End Up with an Innovative Solution

- Serial Ground Faults and Transients: When Perfectly Good Serial Ports Go Bad

I Get a 300 Euro Sales Lesson

A Column of personal opinion by John Rinaldi, Founder and Owner of Real Time Automation.

I spent a few weeks in October in Bologna, Italy. I went for a couple of reasons, none having to do with work, though I did attend a one day technology seminar. That one day seminar is the “official” business reason for my 13 days of drinking Chianti, eating passatelli and spending afternoons sitting in the various Piazzas around the city of Bologna.

At one of the breaks during the seminar I stepped outside to get some air and do some people watching. A man stopped his car at the entrance to the parking lot and called me over. The rest of the story is pretty embarrassing to me so I’m not going to get into the details. A nice way to put it is that we exchanged gifts. When he drove away, I had gifted him 300 of my Euros while he gifted me two nearly worthless coats.

How this came to happen isn’t surprising, at least to me. I am a very trusting, very generous kind of guy. Someday, I’ll write about the time I had a first date with this girl who, in the middle of our dinner, received dire news about her cat. In fact, she had a “cat emergency.” Details omitted again (to protect me) but that “cat emergency” cost me $700.

Yes, I can hear you now. THAT’S BEYOND TRUSTING AND GENEROUS. IT’S DARN RIGHT FOOLISH AND NAIVE. All true. Didn’t someone once say something about how a fool and his Euros are soon parted?

But like everything else that happens to you in life, there is always something to learn. And sales is important to everyone. Most people don’t realize it but everyone needs to learn to sell. You sell your boss on a new project or a raise. You sell your Pastor on the new program for teenagers. You sell your wife on the new hunting rifle. You sell a girl on going out with you.

With that in mind and even though you might not approve of this guy’s business practices he was a good salesman and I came away with three important sales lessons:

Be Fearless - He had absolutely nothing to lose by approaching me. If I walked away at any point he lost nothing.

Establish Rapport - People buy from people they like and people who are like them. He convinced me that we had a lot in common.

Focus on the Result - Everything he did focused on getting to a specific well-defined goal, the money. He established rapport and gave me gifts. How could I refuse my new friend? And once he learned that I still had a few more Euros on me, he went after those too. But by that time I was starting to understand that he might not be such a good friend.

So I’m pretty embarrassed by the whole thing. And yes, it was an awful lot of money but I donated the coats to charity and got a good sales lesson for the money.


Trivia Challenge

· What is the technical term for hitting something until it works?

· What is the only food that includes all the substances necessary to sustain life, including enzymes, vitamins, minerals, and water?

· True or False: Human blood is cheaper than printer ink?

· The city of Rome uses the money that tourists throw into the Trevi Fountain to fund a supermarket for the needy. Approximately how many Euros are thrown in daily ?



Answers located on bottom of page.

S7 Communications

There are really no two bigger names in industrial automation than Siemens and Rockwell. Most of the work I’ve personally done over the years revolves around Rockwell Controllers or devices that work within the Rockwell Industrial Architecture. I’ve worked at Rockwell (actually Allen-Bradley), developed devices to work with Rockwell Controllers and implemented systems at Rockwell customer sites, so I’m pretty thoroughly versed in all things Rockwell including people, products and terminology.

But I, admittedly, don’t know much about Siemens. Since I’m new to Siemens and wanted to write an article about how to successfully deploy “mixed marriage architectures,” Siemens and Rockwell, I spent a lot of my time last month learning about Siemens, Siemens PLCs, and the various communications interfaces for those PLCs. What follows is the information I’ve gathered on Siemens PLC communication interfaces.


The SIMATIC portfolio ranges from small controllers for simple logic tasks to highly complex system solutions. You can get micro controllers, classic Programmable Controllers, PC Controllers, integrated HMI Controllers and Embedded controllers. 

Data Table Organization

Every Programmable Controller vendor organizes its data table differently and it’s been a nightmare since day one. Every time you move from one PLC to another you have to learn the mnemonics of how the next controller structures its data table.

In the Siemens world, a data table is constructed from a set of defined types. The available types vary slightly from controller to controller while the number of data items is vastly different throughout the SIMATIC line.

Here is the list of typically supported data types for a Siemens PLC:













Elements of a data table are accessed by combining the mnemonic for the type with the offset into the data table for that type. IW502 is the 502nd Input Word in the data table while Q10 is the tenth Output Bit.

Communication Interfaces – Protocols & Communication Ports

There are a number of ways to access those internal data items of a Siemens PLC. As with everything else, some PLCs will support one mechanism while others may support two or more different mechanisms. The following sections describe the communications interfaces you may find on a Siemens Programmable Controller.

Serial Communications

Except for one or two models, serial communication is available through an add-on module for all Siemens controllers. The port connects the Controller to barcode readers, printers, operator interfaces and other SIMATIC Controllers. RS232, RS422 and EIA-485 standard (RS-485) can all be used.

Devices can be accessed using a variety of protocols including simple ASCII, Modbus RTU Master/Slave, 3964, and RK512. Modbus RTU, 3964 and RK512, are binary protocols that can be used with devices that need to exchange binary device data with a controller. These protocols can also be used for SIMATIC Controller to Controller communication.

MPI Communications

MPI is a multi-node network used for Programming and SIMATIC Controller to Controller communication. An MPI interface is integrated on CPUs of the modular SIMATIC controllers. There are no MPI interfaces on the S7-1200 and S7-1500.

MPI Communications are proprietary communications that S7 Controllers use to exchange data between each other and other Siemens products. MPI communications use S7 Basic Communication, S7 Communication and Global Data Communication.

The S7 Protocol is a proprietary protocol that facilitates the transfer of data from one Siemens controller to another. The S7 Protocol is a subset of Profibus DP and uses many of the same commands as Profibus DP. In fact, some users have tried to use S7 Controllers as nodes on a Profibus network. This configuration is not supported by Siemens and may not work properly. An adapter is recommended in those applications in which you want to connect the MPI port to a Profibus network.

There are a few off-the-shelf products from third party manufacturers that allow other kinds of devices to communicate on MPI channels.

Profibus DP

Profibus is a smart, sensor-bus technology. Profibus defines an RS485 serial physical layer with special drivers to obtain speeds as fast as 12MB. Slave devices on the system connect to a central controller, usually a SIMATIC S7 Programmable Logic Controller. Once connected, the S7 Controller sends outputs to these devices and receives inputs from them.

Profibus had two huge advantages over other sensor bus technologies. One is speed and the second is data size. 12Meg is really fast. Faster than most people need but generally automation engineers believe that when possible, it’s always best to go faster. Speed has its downside though and that’s cost. It costs more to go that fast. A special ASIC with the communications infrastructure on it (MAC – Media Access Controller) and special transceivers, cabling and connectors. All that makes Profibus devices significantly costlier.

The data size advantage over DeviceNet is significant. Profibus has a frame size of 244 bytes. That’s monstrous compared to 8 byte frames in CAN. Yes, DeviceNet has fragmentation but that eats up so much bandwidth it is hardly useable.

One of the things some people dislike about Profibus is the data representation. All Profibus devices look like a rack of I/O to the controller. That means that a device is a series of slots, each slot with a module in it and all sorts of different size modules. That’s easy for simple devices like I/O devices. You can simply decide that the first slot has a module that is a 16 bit Discrete Input device. The second is 8 bits of Discrete Output and so on. If you also have Analog inputs/outputs, you could set up the device using four slots and four modules. For other non-IO devices, that’s kind of awkward.

Profinet IO

Profinet IO is very similar to Profibus, but not Profibus on Ethernet. While Profibus uses cyclic communications to exchange data with Programmable Controllers at a maximum speed of 12Meg baud, Profinet IO uses cyclic data transfer to exchange data with SIMATIC Programmable Controllers over Ethernet. As with Profibus, a Programmable Controller and a device must both have a prior understanding of the data structure and meaning. In both systems, data is organized as slots containing modules with the total number of I/O points for a system equal to the sum of the I/O points for the individual modules.

Profinet IO uses three different communication channels to exchange data with programmable controllers and other devices. The standard TCP/IP channel is used for parameterization, configuration and acyclic read/write operations. The RT or Real Time channel is used for standard cyclic data transfer and alarms. RT communications bypass the standard TCP/IP interface to expedite the data exchange with Programmable Controllers. The third channel, Isochronous Real Time (IRT) is the very high speed channel used for Motion Control applications. IRT is implemented using a custom ASIC and is not the subject of this paper.

Profinet IO classifies devices into three types; IO-Controllers, IO-Devices and IO-Supervisors. IO-Controllers are devices that execute an automation program. IO-Controllers map IO data from Profinet IO devices into the process image of the controller. In Siemens S7 Programmable Controllers, I/O data, alarms and status data is mapped into the process image in much the same way it is done for Profibus devices. These data values are then available for use by the control program. IO-Controllers must support the following kinds of services:

  • Cyclic Data Exchange - The exchange of data between IO-Controllers and IO-Devices
  • Acyclic Data Exchange - The exchange of Configuration and Diagnostic data
  • Alarms - Alarm data exchange from an IO-Device to an IO-Controller
  • Context Management - Connection processing



Fun Facts

·January 8th, 1836 was the last day in history that the USA had no national debt.

·If the temperature is below 32 degrees outside, bubbles will almost immediately turn into "ice bubbles."

·Norway will allow any student from anywhere in the world to study at their Public Universities completely free of charge.

·The age of a fish can be determined the same way as trees. Fish scales also have one growth ring for every year of age.

·Thanks to recycling, 80-90% of all steel ever produced is still in use today.


  Trivia Answers: Percussive maintenance; Honey; True; 3,000 Euros
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