Hardware Connectivity

Model BB-485PTBR converts unbalanced RS-232 signals to balanced, full or half-duplex RS-485 signals. RS-485 is an enhanced version of the RS-422 Standard allowing multiple drivers and receivers on a two-wire system. The RS-232 port has a female DB9 connector with pins 2(RD), 3(TD), and 5(SG) supported. Pins 7(RTS) and 8(CTS) are tied together. Also pins 6(DSR), 1(CD), and 4(DTR) are tied together, but not passed through the converter. The RS-485 port has an 8-position pluggable terminal block connector.

Baud Rate

Model BB-485PTBR supports baud rates from 300 bps to 115.2 Kbps. In order to change the baud rate, a resistor and possibly a capacitor must be changed. By looking up the selected baud rate on Table 1 the resistor and capacitor value can be determined. Remove R3 and C7 from the printed circuit board. Place new components in the R2 and C6 locations.

Biasing Resistors

The biasing resistors R5 and R7 can also be altered. Model BB-485PTBR comes standard with 4.7K biasing resistors. To change the value of biasing resistors, remove R5 and R7 and replace with new value in locations R4 and R6.

Termination Resistor

The termination resistor is located at R8. A termination resistor can be placed in the R8 location and a jumper wire placed from the terminal location to RD(B). B+B SmartWorx recommends a 100-120 Ohm resistor for termination.

Constant Receiver Enable

The BB-485PTBR is factory-set with the receiver disabled during transmission. It can be set for constant receiver enable. When R9 is removed, the receiver is in constant receive mode (four-wire). When R9 is in the circuit it is in half-duplex mode (two-wire).

Data Line Polarity

The polarity of the two RS-485 lines must be correct. With no data being sent, the RS-232 line should be negative and the RS-485 "A" terminal should be negative with respect to the "B" terminal. If your equipment uses a "+" and "-" naming scheme, in most cases, the "A" line will be connected to the "-" and the "B" line will be connected to the "+".

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