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OMG-CONV-485-ISO Page 7
RS-485 Two-Wire and RS-485 Four-Wire Slave Installation
If your application is 2-wire Half-Duplex RS-485 or 4-Wire Full-Duplex RS-485
Slave that requires control of the transmitter, the OMG-CONV-485-ISO provides 3
methods of control.
The first method is ‘Auto Enable’ which uses an onboard microcontroller to
sense the baud rate and control the transmitter enable. This mode assumes a
minimum baud rate of 1200bps and a word length of 10 bits. This would be the
preferred method, as it requires no software in order to work. This mode should
be used with line termination since the transmitter is driving the line during
transmission.
The second method is ‘RTS Enable’ which uses the Windows based serial
drivers to control the UART modem control signal RTS. RTS is connected to the
transmitter enable circuit and thru software control, provides an efficient method
of line-turnaround. This abstracts the application from the low level control and
simplifies application development. This mode should be used with line
termination since the transmitter is driving the line during transmission.
The last method is ‘Data Enable’ which uses the transmitted data stream to
enable the transmitter and requires no special software drivers. When the
transmitted data is a low (0), the transmitter will be enabled to drive the line.
When the transmitted data is a high (1), the transmitter will be tri-stated relying
on the line biasing resistors to pull the line to the proper state. Due to this,
termination must not be used and thus data rate and cable length are reduced
accordingly.
The choice of which method to use is simple. If your data stream permits the use
of the ‘Auto Enable’ mode always use this option as it permits the simplest
installation. If ‘Auto Enable’ is not an option and you are using the OMG-CONV-
485-ISO in a Windows based system, setup the transmitter control for ‘RTS
Enable’ and install the SeaCOM software. Again, if ‘Auto Enable’ is not an
option, and your system is not Windows based or if the port you are converting
is not PC based, use ‘Data Enable’.
If ‘RTS Enable’ mode is chosen, start by choosing Install Software at the
beginning of the CD. Choose Asynchronous COM: Port Software, SeaCOM.
Continue with the installation until the “Setup Type” dialog, as shown below,
appears. Choose “Typical (complete)” to install the software including drivers
and applications. A Start Menu Program Group, “SeaCOM” will be created and
WinSSD can be started from there.