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. The second jumper "RX" of JP4 determines the receiver mode. When the jumpers are placed in the “485” position, the "echo" is turned off. This is achieved by disabling the receiver when the transmit driver is enabled. Placing this jumper in the “422” position will hold the receiver enabled at all times. More information on RS-422 communications can be found in B&B’s free RS-422/485 Application Note (available on our websites).
RS-422 and RS-485 Termination
A120Ω termination resistor has been provided for the RS-422/485 receiver. Note that termination should only be used in systems with both high baud rates (>19200) and over several thousand feet of cable. If a value other than 120 Ω is desired, space for a through- hole resistor has been provided on the board adjacent to the surface mount termination resistor. This through-hole termination resistor is labeled as RTAB while the surface mount termination resistor is labeled as RTAA. See B&B’s free RS-422/485 Application Note, Termination section, page 16, for more information (available on our websites).
RS-485 Network Biasing
Biasing is required on an RS-485 network to hold the network in the marking state between transmissions. The 3PCIOSD1x RS-485 receiver is biased with a 4.7 KΩ pull-up resistor (R28A) on the Receive Data B line and a 4.7 KΩ pull-down resistor (R29A) on the Receive Data A line. These values are usually adequate for networks without termination and small numbers of nodes. For more complex networks, the user must calculate the required value and replace these resistors. Space for through-hole resistors has been provided over the top of the surface mount components placed at the factory. The through-hole resistor for the pull-up is marked as R28B and the pull-down is marked as R29B.
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4 X Baud Rate Option
Baud rates higher than 115,200 are possible with the 3PCIOSD1x card in RS-232, 422, or 485 mode. Jumper JP3 controls the clock frequency supplied to the UART. By moving this jumper to the “*4” (left) position, the clock frequency is increased from 1.8432 to 7.3728 MHz. This multiplies all UART baud rates by 4 times. For example, if the software is set for 57.6K baud, the actual baud rate will be increased by a factor of four to 230.4K baud. In many systems, these higher baud rates can improve throughput significantly. However, remember that baud rates and actual throughput are only proportional if the system can keep up with the communications, otherwise increasing the baud rate effectively only increases the idle time between characters.
Installing the Card
1.Ground yourself by touching the metal chassis of the computer to discharge any static electricity.
2.Turn the power to your computer off and unplug the power cord.
3.Remove the cover of the computer.
4.Locate a free PCI expansion slot.
5.Remove the expansion slot cover. Save the screw for installation of the 3PCIOSD1x card.
6.Install the card into the unused slot. Be certain that the card is inserted completely (fully seated) in the slot.
7.Secure the card with the mounting screw from step 5.
8.Replace the cover, plug in the power cord, and power up the system.
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