RS-485 Mode, RS-422 Pinouts, DB25M, Name | B&B Electronics 3PCIOSD1A

RS-422 Pinouts

The RS-422 mode supports 2 channels (transmit and receive).

Name	Description	Direction	DB25M
			Pin

Shield	Shield	------	1
TD (A) −	Transmit Data A	Output	2
RD (A) −	Receive Data A	Input	3
GND	Signal Ground	------	7
TD (B) +	Transmit Data B	Output	14
RD (B) +	Receive Data B	Input	16

Table 2: RS-422/485 Pinouts

With 2-wire RS-485 mode operation, your connection cable must jumper TD(A) to RD(A) and TD(B) to RD(B). Connect from TD(A) and TD(B) to the Data A(−) and Data B(+) wires of your RS-485 network.

Note that the EIA RS-422 Specification labels data lines with an "A" and "B" designator. Some RS-422 equipment uses a "−" and "+" designator. In almost all cases, the "A" line is the equivalent of the "−" line and the "B" line is the equivalent of the "+" line. More information on RS-422 communications can be found in B&B’s free RS-422/485 Application Note (available on our websites).

RS-485 Mode

To configure the port for RS-485 mode, 8 jumpers must be set/checked. The following settings configure the port as RS-485:

1.Set four jumpers of JP2 (A-D) to the "422/485" (right) position.

2.Set the top jumper "TX" of JP4 to the “485” (right) position.

3.For 2-wire operation, set the second jumper "RX" of JP4 to the "485" (right) position (half duplex). For 4-wire operation, set it to the "422" position (full duplex, Receive enabled)

4.Set the JP1 Control jumper to select the type of RS-485 transmit driver control, automatic Send Data (SD) or RTS . Select SD control unless you are sure that your software requires RTS control.

Documentation Number 3PCIOSD1x-3903m Manual

Chapter 2

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5.The bottom jumper "Termination" of JP4 switches the 120 Ω 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 B&B’s free RS-422/485 Application Note, Termination section, page 16, for more information (available on our websites).

Figure 4 shows the jumper configuration to set the port for 2-wire RS-485 mode with *1 clock enabled.

	JP2	422/ 485		JP1
A 232		422/ 485	CONTROL
B 232		422/ 485	SD		RTS(232)
C232		422/ 485
C232		422/ 485
D232		422/ 485

	MODES
TX 422	485		JP3
RX 422	485 JP4		JP3
I N	OUT	*4	*1
TERMI NATI ON			CLOCK

Figure 4. RS-485 Mode Jumper Settings RS-485 Pinouts

The pinouts in RS-485 mode are the same as those listed in Table

2:RS-422/485 Pinouts on the previous page.

Explanation of RS-485 Operation

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 3PCIOSD1x 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 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 de-assert to disable the driver. To select SD control for the port, place JP1 jumper in the “Send Data” (left) position. Place this jumper in the “RTS(232)” (right) position for RTS control.