User Manual

Documentation No.3PXCC1x-2x-3903 37
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph [phone] – Fax [phone]
Figure 4 shows the jumper settings for RS-485 mode.
Figure 4. RS-485 Mode Jumper Settings
RS-485 Pinouts
The pinouts in RS-485 mode are the same as those listed in
Table 7 for RS-422 mode.
RS-485 Operation
RS-485 mode requires that the driver be enabled and
disabled as needed, allowing two or four-wire communications. To
set up the 3PXCC1A Serial Card up for two wire mode you can
simply jumper TD(A) to RD(A) (for your Data “-“) and TD(B) to RD(B)
(for your Data “+”). Use the Signal Ground for your return path as
the third wire in this configuration. For four wire mode, simply use all
four data lines and signal ground for a return path.
The 3PXCC card provides two methods of
enabling/disabling the driver: automatic send data (SD) control and
request to send (RTS) control. With automatic SD control, the driver
is enabled when data is transmitted. It remains enabled for the
transmission time of one character after data transfer is complete.
The SD circuit automatically adjusts its timing to the baud rate of the
data. With RTS control, software must set the RTS bit to a logic 1 to
enable the driver and logic 0 to disable the driver. To select SD
control, place jumpers JP1D and JP2D in the SD position (left).
Place these jumpers in the RTS position (right) for RTS control.
38 Documentation No.3PXCC1x-2x-3903
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph [phone] – Fax [phone]
The receiver can also be enabled and disabled, a useful
feature in two-wire communications to prevent the transmitted data
from "echoing back" on its own receiver. Jumpers JP1C and JP2C
control this feature for Port 1 and Port 2. If these jumpers are placed
in the RS-485 position, the "echo" is turned off. This is achieved by
disabling the receiver when the driver is enabled. Placing these
jumpers in the RS-422 position will hold the receiver enabled at all
times. More information on RS-485 communications can be found in
B&B Electronics’ free RS-422/RS-485 Application Note.
RS-422 and RS-485 Termination
A 120Ω termination resistor has been provided for the RS-
422/485 receivers. To enable the termination provided, place JP1E
for Port 1 or JP2E for Port 2 in the Rt IN position (right). If you do not
need to use termination, place the jumpers in the Rt OUT position
(left). Termination should only be used when very long wiring 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 3PXCC cards by
adjusting JP1G and JP2G. This multiplies the clock speed supplied
to the UARTs by 4 times to 7.328MHz for ports 1 and 2 respectively,
allowing data rates up to 460.8K baud in RS-422 and RS-485
modes. Note: RS-232 does not support these extended baud rates.
To use the higher clock rate, place the jumper for the
appropriate port in the *4 (left) position. The jumpers are shipped
from the factory in the standard clock (right) 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.