Isolated RS422 Interface Circuit and Layout

 Isolated RS422 Interface Circuit
Figure 1 shows the circuit diagram of RS422 interface. Connector K1 is linked to the serial port of the PC, power to the PC side of the circuit is derived from the signal lines DTR and RTS. Positive supply is derived from RTS and negative supply from the DTR line. Optical isolation is achieved by optocouplers U1 and U2. Optocoupler  U1 provide TXD line isolation while optocoupler U2 provide RXD line isolation.

The other side of the isolator carries TTL levels. This side is powered by an unregulated dc supply between 9V and 12V ac/dc adapter. IC U5 provide 5V regulated output and ICs U3 & U4 provide the RS422 bus interface. The TXD and RXD lines status are provided by data indicating LEDs. The interface has been tested at the baud rate of 19.2k baud.

The connector K3 provide the RS422 bus signals plus the un-regulated dc supply output. An RJ45 connector can be used to provide the signals & power to the other end of the RS422 bus.
The PC RXD pin can only receive the data when the RS232 handshake lines are set accordingly,
RTS = 1  (at +ve supply level)
DTR = 0  (at -ve supply level)
For printing the pcb files on HP laser printer download the file and run the Laser.bat file after extracting from the zip file.
Figure 2 & 3 shows the component layout of the RS422 single sided pcb and the track patterns respectively.

Component details of the project.
1 1 B1 BRIDGE 1A/100V (DIP TYPE)
2 3 C1,C2,C3 100nF CERAMIC
3 1 C4 10uF 16V
4 1 C5 470uF 25V
5 2 D1,D2 1N4148
6 2 D5,D3 LED RED 3mm
7 2 D4,D6 1N4003
10 1 K3 SIP CON 8 WAY
11 4 R1,R2,R5,R6 1K
12 1 R3 4K7
13 1 R4 470R
14 4 R7,R8,R9,R10 10R
15 2 R13,R11 120R
16 2 R14,R12 680R
17 1 U1 6N137
18 1 U2 CNY17-3
19 2 U3,U4 SN75176B, MAX485
20 1 U5 LM7805

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