RS-422 and RS-485 Termination
A 120Ω termination resistor has been provided for the RS- 422/485 receivers. To enable the termination provided, the termination jumper should be placed in the Rt IN position (left). If you do not need to use termination, place the jumper in the Rt OUT position (right). Termination should only be used when very long cable runs are used with high baud rates. For example, with most cables which are 4000 feet or shorter and have baud rates at 19.2K baud or lower, termination is not required. Note that if the termination is enabled (IN), the biasing of the
High Speed Mode
High data rates can be obtained with the 3PXCC4A cards by adjusting JP16B and JP17B. This multiplies the clock speed supplied to the UARTs by 4 times to 7.328MHz for all ports, allowing data rates up to 460.8K baud in
To enable the X4 clock, simply move the shorting jumper from JP16B (*1) to JP17B (*4). This jumper is shipped from the factory in the standard clock (*1) position.
Note that serial software is not aware of the change in oscillator frequency. For example, in the × 4 position, setting the baud rate to 57.6K baud in software will result in a actual baud rate of 230.4K baud. It is important to note that increasing the baud rate may not increase actual throughput. In heavy multitasking cases or with a slow computer, the computer’s inability to respond to interrupts quickly enough will cause large idle spaces between characters. Increasing the baud rate to this point will not increase actual throughput.
38 | Manual Document# 3PXCC4A3001 |
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