Interfacing Medical Equipment to the Profibus DP Industry Standard Fieldbus

Péter Várady

Dept.of Control Engineering and Information Technology, Technical University of Budapest, Hungary
1111 Budapest, Műegyetem rkp. 9, Hungary E-mail: varady@iit.bme.hu

Abstract

  In this article a new concept of system integration in the filed of medical applications will be presented. Today medical communication standards exist only for the higher level of medical care, like various databases and hospital information systems. Low level communication tasks between the bedside diagnostic devices is not yet standardized, so devices of different vendors cannot be interconnected into a common medical patient monitoring system. Our approach uses the technique and the off-the-shelf components of an industry standard, open fieldbus. The system is so planned that also the already existing devices are connectable with external link modules. The second part of the article describes the design concept of the link module.

Keywords: medical monitoring, medical instruments, fieldbus, Profibus DP

Foreword

  The rapid development of information technology enables the direct use of newest results of biomedical researches [1]. One of the most important areas is the modeling of physiological processes, which helps us to understand better the human body. The exact the mathematical model of a physiological process, the bigger is the required computer time to evaluate the raw measured data due to diagnostic purposes [2]. It is very important to emphasize the need of interconnecting various diagnostic equipment into a common communication network.

  The communication between medical devices and applications is more on less standardized [3], but in case of the bedside monitoring systems there are only vendor-dependent and closed communication protocols on the scene. In these applications the various bedside medical equipment are to be networked at a lower level of the communication hierarchy. This requires a dynamic, real-time, deterministic, fault-tolerant and secure data exchange. The 802.x standards, which are widely used in the physiological centers and hospitals, are lack of some or more of these features [4]. Since 1992 IEEE has a draft proposal of the Medical Instrumentation Bus (MIB) [5]. The final standard is not yet approved. Another problem is, that MIB is very poor hardware supported, and even if MIB would be already approved, it would take many years till vendors will offer their MIB interfaced devices. Although these problems are existing, we tried to find another convenient, affordable and standardized way to interconnect bedside equipment.

The motivations

  Let's have a look at the key requirements of bedside patient monitoring [6]:   As it can be established, to fulfill all of these requirements we must define the lower layers of the OSI model. It can be noticed that the wanted features are very similar to the ones, which are existing in the field of the industrial communication systems. Our basic idea was to use a standardized an industry standard serial digital fieldbus transmission technology, which satisfies all of the expected requirements. These standards are supported since many years with the appropriate hardware and software components. So various medical devices can be interfaced to a fieldbus using an existing know-how, with off-the-shelf components.
  The interfacing can be done in two different ways: a device can itself contain the interface or an external interface module can be used. The last approach is more economical, since already existing equipment can also be easily interfaced such way. Almost all of the bedside devices have a quasi-standard analogue signal output and these analogue outputs can be measured and interfaced onto the fieldbus with the external link modules. After it the data can be transmitted in a digital form to the central monitoring station (a desktop PC with a GUI visualization) through the fieldbus data link (FDL).

  To introduce the ideas described above, we decided to realize a distributed demonstrator system based on the open fieldbus European Standard EN 50170 Vol. II, Profibus. The Profibus DP is a vendor independent, open field bus standard for various applications and originally based on Part I and III of the German DIN 19245 standard [7,8]. Profibus DP was developed for its simple standardized user interface and its main goal is to realize a simple, but fast cyclical data transfer, upto 246 bytes/data per message and up to 12 Mbit/s between the various slave and master devices connected to the bus. The transmission media can be either a single twisted pair or optionally an optical fiber at higher speeds. Today Profibus DP is the market leading fieldbus technology in Europe. The detailed features of Profibus DP can be found in the standard mentioned above. For a brief technical reference check the Profibus web site [9].

  This article will describe an affordable, low-cost external Profibus DP slave module, which allows externally interface already existing devices to the fieldbus. The device to be Profibus interfaced can be connected via a simple asynchronous RS232 serial line with the external fiieldbus interface module. In our application a single-card industrial PC was linked to the Profibus DP with the external link module. The PC measures and preprocesses various analog signals generated by bedside medical equipment [10].

Design considerations

  The DP system based on the layered ISO OSI model. The first layer of the physical transmission media is the twisted pair RS485, which is a symmetrical voltage driven line. The standard allows 32 devices in a bus-segment and 4 segments in a system. The second layer (media access layer) describes a telegram oriented master-slave protocol, where multiple masters are allowed, and the masters using deterministic, slot-time driven token-passing mechanism to arbitrate on the bus (FDL - Fieldbus Data Link protocol). The data transmission is CRC protected and has a base level network management functionality. The remaining OSI layers are not implemented, but there is a standardized DP user interface (USIF). The user interface has the following features: The possible ways to implement the DP protocol: The pure software implementation requires quite a big programming effort and CPU time. The microprocessor or microcontroller based hardware card would require also a big effort, and the debugging and testing of correct functionality would take even months of time. In the last years some vendors are offering tested ASIC cores to fully implement either the FDL or both the FDL and DP protocol parts. This solution requires the least development effort and offers a low-cost price. One of the Profibus ASIC vendors is Siemens, which actually offers a family of ASICs. The line of SPC cores (Siemens Profibus Controller) implement various parts of the Profibus standard. Our choice was the ASIC chip SPC3, which fully implements the whole DP slave protocol [11].

The SPC3 ASIC core

 The SPC3 core is designed to be an intelligent CPU periphery and to able to interfaced to various kind of CPUs and MCUs. It requires only a few external components (some resistors, an oscillator and an RS485 physical interface) . The ASIC has an on-chip 1,5 kB dual ported RAM, and fully implements the DP protocol. In order to the correct functionality at first the internal parameter and mode registers and the buffer management of the ASIC must be initialized. The DP related events generate an interrupt, so the host CPU can read or write the dual port RAM, in order to the required functionality. These events are:

The host CPU

 The host CPU is the CPU of the external interface module, which handles the ASIC core, and realizes the data transfer between the interfaced device and the ASIC. The data is transnmitted between the interfaced device and the the external module via a standard serial RS232 line. To allow as high performance as possible baudrates up to 115,2 kbit/s are required. The MCU PIC16C63 from Microchip [12] was chosen to realize the host CU. This microcontroller has optimal feature: The development of the PIC16C63 code was done with the help of an in-circuit emulator, and the whole code size is a bit more than 2 kword.

The parameters of the DP slave (modes, buffer sizes, station address, timeouts etc.) can be saved into and loaded from an I2C interfaced serial non-volatile memory. This enables the end-user minimal efforts in the high level application.

The high level application interface

 The target of the work is to interface a device via its RS232 onto the Profibus DP network. In other words the interfaced device can reach the DP services via an RS232 interface. In our application the interfaced device is a single-card industrial PC. This PC runs a 32 bit Microsoft Windows operating system. To achieve a flexible surface to various kind of applications, we decided to realize a dynamic link library (DLL), which enables to reach the DP services from any programming environments. As the standard serial line is used to connect the external DP module, no device drivers are required.

 The DP DLL offers a very simple interface for the applications. Generally the applications should create a polling thread in order to poll the status of the external slave, so the DP bus can be monitored. The DLL supports an interface to:

References

  1. Benyó Z.: Computer Analysis of Physiological Systems. A New Event Recognition Method. System Analysis and Simulation, Gordon and Breach Publ. 1995, Vol. 18-19. pp. 733-745.
  2. Z. Benyó - L. Czinege: Computer Analysis of Dynamic Systems with Application in Physiology. Proc. 15th World Congress of IMACS on Scientific Computation, Modeling and Applied Mathematics. Berlin 1997, Vol. III. Pp. 663-668.
  3. S. Sengupta: Computer Network in Health Care, The Biomedical Engineering Handbook, IEEE Press, 1995, ISBN 0-8493-8346-3, pp. 2642-2649.
  4. http://stdsbbs.ieee.org/groups/mib/tech/1073vsEthernet.html
  5. http://stdsbbs.ieee.org/groups/mib/main.html
  6. P. Várady: Distributed communictaion in Biomedical Applications, IEEE Hungarian Section
  7. PROFIBUS Standard Process Field Bus DIN 19245-I-II.
    Translation of the German Standards, Profibus Nutzerorganisation e.V., 1991
  8. PROFIBUS Dezentrale Peripherie (DP) DIN 19245-3 Standard
    Deutsche Elektrotechnische Komission im DIN und VDE, Ausgabe 1993-10
  9. http://www.profibus.com
  10. S.M. Szilágyi: Fast Biological Signal Analysis and Real-time Processing, Proceedings of the Symposium on Fieldbus Systems and Application Technics IEEE Hungarian Section, 1998., ISBN 963-421-547-5, pp. 45-50.
  11. SIMATIC NET SPC3 Siemes Profibus Controller User Description, 1996/10 SIEMENS AG, 6ES7 195-0BD00-8BA0
  12. The Embedded Controller Data Book, Microchip Corporation, 1996.