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1-4 Introduction
1.5 RS-485 on OMR
The OMR improves the RS-485 capability for minimizing the user‘s cost. On each OMR module, a half-duplex
RS-485 transceiver is used to communicate with other modules. A single twisted pair of wires, which provides
standard differential transmission, is used to transmit and receive data between modules. The high input
impedance of each OMR receiver allows up to 128 OMR modules on the same RS-485 bus without using a
signal repeater.
The maximum transfer rate of OMR is 115.2Kbps, which is lower than the maximum speed of the RS-485
standard. The slew-rate limiter on every RS-485 transceiver of OMR is very useful for transmitting error-free
data, minimizing EMI, and reducing reflections caused by improperly terminated cables.
The OMR on a network may not use the same power supply. Therefore, the voltage difference between
grounds of the modules may exist.
Excessive output current and power dissipation caused by faults or by bus contention are prevented by the
current limiter and the thermal shutdown circuitry inside the OMR.
1.6 OMR RS-485 Network Configurations
OMR-6000 series is designed under RS-485 multi-drop network architecture. Up to 256 OMR modules can be
controlled in a multi-drop network. The limit of 256 is due to command code. The network can be connected
by simple topology (Figure 1-1) or branch topology (Figure 1-2) or free topology (Figure 1-3).
The OMR-6520 and OMR-6510 are the two basic communication modules to construct a RS-485 network. The
OMR-6520 is a RS-232 to RS-485/RS-422 converter. The OMR-6520 is used to build a RS-485 port for the host
computer by converting standard RS-232 signal into RS-485 signal.
The OMR-6510 is the RS-485 signal repeater, which is used to extend or to lengthen the network distance. An
OMR bus can connect up to 256 modules, each segment is up to 128 modules. Whenever the numbers of the
modules excess 128, the repeater should be used. In addition, the length of a standard RS-485 bus is up to
4000 feet, the repeater should be used whenever the length of a signal bus is more than 4000 feet.
Figure 1-1 Simple Topology
Host
ND-6520
RS-232/RS-485
Converter
OMR Modules
RS-232
RS-485 bus
OMR-6520
RS-232 to RS-485/RS-422
Converter
Terminator