38 Manual Document# 3PXCC4A3001
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445
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 RS-485 network is
altered and the value of the bias resistors will likely need to be
changed somewhere on the network. More information on
termination and biasing can be found in B&B Electronics’ free RS-
422/RS-485 Application Note.
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 RS-422 and RS-485 modes. Note
that RS-232 does not support these extended baud rates.
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.