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
| | | Resistor Value | | | Capacitor Value |
Baud Rate | Time (ms) | | R15 (ohms) | | 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 | |
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