Phytec miniMODUL-537, Hardware Manual

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miniMODUL-537
40
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PHYTEC Meßtechnik GmbH 2001 L-006e_2
6.6.2 RS-485-Interface
The transmission with RS-485 standard allows a maximum of 32
participants for a two wire line. In comparison to the RS-232 interface
higher baud rates and line length are possible. The maximum trans-
mission rate of the SAB80C537 with 1 Mbit/sec for a system-clock
frequency of 12 MHz can be achieved. Since 32 participants can be
connected to the two wire line for this standard more control and
protocol management is required than for the RS-232 interface. The
transmission with RS-485 standard operates with a two wire bi-
directional line with alternating directions. Both ends of the line are
terminated with resistors. The symmetric data-transfer of the two wire
line considerably improves the immunity against interfering.
The cross linkage of these modules in the industrial environment
requires specific knowledge of the occurrence of the special problems.
Interferences from electromagnetic devices in the surrounding,
potential differences of different stations and the effect of ground
loops have to be considered. For such purposes the galvanic separation
of processor and network are advisable.
6.7 Changing from RS-232 to RS-485 Standard
The RS-232 interface cannot be modified to RS-485 without
additional software. For such a project it is also important to know if
the participants are operating in a master-slave or multi-master
network. In addition to the data-inputs also the inputs of the driver
chips of all stations have to be controlled in a skillful way. Therefore
PHYTEC has developed a software with the designation PHYNET for
the communication with RS-485 interfaces. With this software all
modules with RS-485 interface can be interconnected with a network.