Reference Number: INTAS-97-0606
INTAS logo Development of an Intelligent Sensing Instrumentation Structure I.S.I.S.
System Design of ISIS
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Features of ISIS higher level | The implementation of ISIS units
Data Bus alternatives
The selection of the Data Bus type (electrical characteristics) and its communication protocol is a very essential issue. The system has many implicitly master devices. The Data Bus must support bi-directional half-duplex multi-point communication and continue to operate even in a heavily noisy (industrial) environment. There are two standardized interfaces suitable for the system: TIA/EIA-485-A and CAN.
TIA/EIA-485-A
TIA/EIA-485-A is an electrical standard, specifying balanced drivers and receivers. It is the only TIA/EIA standard that allows for multiple driver operation at this time. This fact allows for multipoint (party line) configurations. The standard specifies a bi-directional (half duplex), multipoint interface.
Key Features are:
  • Balanced Interface
  • Multipoint Operation
  • Operation From a Single +5V Supply
  • 7V to +12V Bus Common Mode Range
  • Up to 32 Transceiver Loads (Unit Loads)
  • 10 Mbps Maximum Data Rate (@ 40 feet)
  • 4000 Foot Maximum Cable Length (@ 100 kbps)
CAN
CAN is a complete DTE/DCE balanced interface standard that provides a serial communications protocol to support distributed real-time control applications. With its electrical characteristics similar to the 485, it offers high data security implemented by standard integrated circuits. The transfer protocol includes all bit timing, frame formatting, message identification, data transmission, acknowledgement, and error checking definitions. CAN units are connected as "plug and play" devices on the bus and CAN automatically handles the entire data transfer function through the following steps:
  • Priority-based bus contention resolution
  • Data transmission
  • Error detection
  • Automatic re-transmission of failed messages
  • Message delivery acknowledgement
  • Automatic shutdown of failed nodes
The current CAN specification is Version 2.0 (1991, Robert Bosch GmbH) and is an open document and freely available on the Internet, it is also documented in ISO-11898. Information may be found on www.kvaser.com.
Comparing the two alternatives, the CAN interface is more suitable for the extensive ISIS of Figure 2.

Nevertheless, in the current phase of the project, the objective is the development of a minimal ISIS (one Measuring Mode, no User Terminals) for the execution and test of the intelligent neural network functions. For this, a simple multiplexed 458 bus, controlled by the Central Computer, is adequate. By this concept, we implemented the Portable Tester and the Measurement Module that are presented below.