M Asim Khan    asimkhan@sat.net.pk

This interface circuit provides electrically isolated RS422 communication interface to the PC serial port the isolation circuit protect the PC from direct connection to hazardous voltages.

Figure 1: Circuit Diagram of Isolated RS422 Interface
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 PCBs.zip 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.

Figure 2: Component layout of the RS422 PCB

Figure 3: Track patterns of the RS422 PCB

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
    8       1    K1                        DB9 R/A PCB PLUG
    9       1    K2                        DC JACK SOCKET
   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

                                                                                                                                                                       30 Jul 2002