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EM1500 User’s Manual
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Usually, the master has ultimate control of who speaks, and when. The master can transmit whenever it
knows that a slave is not currently transmitting. A slave can only transmit when the master has given it
express permission to do so.
The EM1500 should be used as the master. All other nodes should be configured as slaves in the sense that
they only transmit when specifically addressed by the EM1500 that is acting as master.
Messages are generally limited to a fixed maximum size. The nature of the medium does not lend itself to
long “monologues” from any one device, including the master. Typically, the master will issue a short
command packet, which is addressed to a particular slave. The slave will then respond within a fairly
tightly defined interval.
The EM1500 provides some software support for typical RS485 protocols, where messages are less than
an upper limit of 1020 bytes, and the EM1500 is the master device. Since there are many different RS485
protocols, many of which are proprietary, the EM1500 only provides the most general support. Successful
implementation of a particular protocol will require the correct configuration of several other items. The
most important of these are the specification of the transmitter enable discipline (which will usually be
“Rx Idle” in the Serial tab) and the packetizing protocol (in the Protocol tab).
Users of PC COM port redirector software will usually not be able to use the RS485 port successfully.
This is because of limitations in the underlying RFC2217 protocol, which does not have any concept of
packetizing. Full use of the RS485 port requires either another EM1500 or PC software written to take
advantage of the Rabbit extensions to RFC2217. Another EM1500 may be used in client/server mode to
implement a protocol converter. The conversion may be between any of the serial ports, RS232 or RS485,
on the client and the RS485 port of the server (or vice versa).
Implementation of a specific RS485 protocol will generally require in-depth knowledge of the protocol, as
well as thorough understanding of the configuration options outlined in this manual.
Modbus RTU is a widely used protocol, which is a good candidate for use of the RS485 port. We use this
protocol as an example of how to configure the RS485 port for compatibility with an existing, popular pro-
tocol.
Summary of Contents for EM1500
Page 14: ...10 www rabbit com Introduction...
Page 22: ...18 www rabbit com Getting Started...
Page 76: ...72 www rabbit com EM1500 Configuration...
Page 90: ...86 www rabbit com EM1500 Specifications...
Page 104: ...100 www rabbit com Serial and TCP Protocols...
Page 118: ...114 www rabbit com EM1500 FAQ...