Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
Page 19 of 137 Manual Documentation Number: <DocNumber>
www.bb-elec.com/
www.bb-europe.com/
One technique for accomplishing this is to use a retriggerable timing
circuit, as shown in the following diagram.
Figure 11: Send Data Control
The timing circuit uses a resistor and capacitor to set the retriggering
time constant. The circuit enables the driver when it detects the leading
edge of the first data pulse. As long as data is being received by the
converter the timing circuit is continuously retriggered, maintaining the
enable signal to the RS-485 driver. When the data stops (and the data
signal returns to the idle state) the timing circuit releases (disables)
the driver after a configurable period of time, typically one character
length (ten bit lengths).
Since the length of time required for one character varies depending on
the baud rate of the system, the time period of the timing circuit must
be configurable. When using a retriggerable time circuit the timing
circuit can be configured by changing the R and/or C values.
Some RS-485 devices come from the factory with R/C values preset for
9600 baud. Others have switch selectable presets.