B&B Electronics 3PCISD4B User Manual

 

Documentation Number 3PCISD4x-3903 Manual 

Chapter 2 

9 

5.

 

Set the fourth jumper of JP3 (D) to select the type of RS-485 
transmit driver control, RTS or Send Data.  Select SD control 
unless you are sure your software requires RTS control. 

6.

 

The bottom jumper of JP3 (E) switches the 120

Ω

 receiver 

termination resistor in or out.  Typically this resistor is not used.  
In some cases using high baud rates and very long cables, 
termination is needed.  See our RS-422/485 Application Note 
(available on our websites). 

Configure each port as above using the table in Appendix B. 

The pinouts in RS-485 mode are the same as those listed in Table 8 
for RS-422 mode.   

In RS-485 mode, the transmit driver must be enabled to transmit, 
and set to a high impedance (tri-state) mode at the end of 
transmission.  In two wire (half duplex) mode, the receiver is 
disabled during transmit, and enabled when not transmitting. 

The 3PCISD4 card provides two methods of enabling/disabling the 
transmit driver:  automatic Send Data (SD) control and Request To 
Send (RTS) control.  With automatic SD control, the transmit driver 
is enabled when data is sent.  The driver remains enabled for the 
transmission time and ten data bits after data transfer is complete.  
The SD circuit automatically adjusts its timing to the baud rate of the 
data.  With RTS control, software must assert the RTS line to enable 
the driver and disassert to disable the driver.  To select SD control 
for Port 1, place the fourth jumper of JP3 (D) in the SD (right) 
position.  Place this jumper in the RTS (left) position for RTS control. 

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 on JP3 (B) 
determines the receiver mode.  If 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. 

Refer to the Jumper Mode Table in Appendix B for all ports.  More 
information on RS-485 communications can be found in our RS-
422/485 Application Note (available on our websites). 

10 

Chapter 2 

Documentation Number 3PCISD4x-3903 Manual 

An 120

Ω

 termination resistor has been provided for the RS-422/485 

receivers.  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.  The termination resistors are labeled 
RTAB, RTBB, RTCB and RTDB for ports 1, 2, 3 and 4 respectively.  
See our RS-422/485 Application Note for more discussion on 
termination (available on our websites).  

Baud rates higher than 115,200 are possible with the 3PCISD4x 
card in RS-232, 422, or 485 mode. Jumper JP5 controls the clock 
frequency supplied to the UARTs. 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 57.6 Kbaud, the actual baud rate 
will be increased by a factor of four to 230.4 Kbaud.  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.