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485DSS3298 Manual 5
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
SD control automatically enables the RS-485 driver by sensing
the leading edge of the first character transmitted to the RS-232
port. After transmitting the last character, the send data timer circuit
waits one character time (1 millisecond at 9600 baud) before
disabling the RS-485 driver.
There are two components in the timer circuit, a resistor (R15)
and a capacitor (C10), that make up the send data control circuit.
These components are factory selected for 9600 baud, allowing the
send data control to operate at 9600 baud or higher. These two
components disable the RS-485 driver approximately 1 millisecond
after transmitting the last character. To change to a baud rate lower
than 9600 baud, or to configure the send data control to match a
specific baud rate of the RS-232 device, remove R15 and insert the
value specified in Table 3 into the R21 position. Use Figure 2 to
locate R15 and C10. NOTE: The SD timing circuit does not control
the RS-485 driver when the 485DSS sends reply messages to the
host.
Table 3. Component Values For Send Data Timing
Baud Rate Time (ms)
Resistor Value
R15 (ohms)
Capacitor Value
C10(microfarads)
1200 8.33 820K .01
2400 4.16 430K .01
4800 2.08 200K .01
9600* 1.04* 100K* .01*
19200 0.520 56K .01
38400 0.260 27K .01
56700 0.176 16K .01
115200 0.0868 8.2K .01
* Factory Default