PC  RS485  Interface

M Asim Khan,   asimkhan@sat.net.pk

This interface circuit provides electrically isolated RS485 communication inteface to the PC serial port the isolation circuit protect the PC from direct connection to hazardous voltages.

Figure 1: Circuit Diagram of Isolated RS485 Interface
Figure 1 shows the circuit diagram of RS485 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. The RTS line is also used to control the data direction of RS485 driver IC U4.
Optical isolation is achieved by optocouplers U1, U2 and U3. Opto U1 is used to control the data direction of U4 opto U2 provide RXD line isolation while opto U3 provide TXD line isolation.
The other side of the isolator carries TTL levels. This side is powered by an unregulated dc supply between 9V and 18V dc. IC U5 provide 5V regulated output and IC U4 provide the RS485 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.
For Data Reception            RTS = 1  (at +ve level)
For Data Transmition          RTS = 0 (at -ve level)
DTR line is always set to 0  (at -ve level)

Figure 2 & 3 shows the component layout of the isolator pcb and the track patterns respectively.

Figure 2: Component layout of the Isolator PCB

Figure 3: Track patterns of the Isolator PCB

Component details of the project.

 1       4    C1,C2,C3,C6       100nF
 2       1    C4                10uF 16V
 3       1    C5                470uF 25V
 4       3    D1,D2,D3          1N4148
 5       2    D4,D5             LED RED 3mm
 6       2    D7,D6             TRANSIL 6.8V
 7       1    D8                1N4003
 8       1    K1                DB9 R/A PCB PLUG
 9       1    K2                PCB TERMINAL BLOCK 4 WAY
10       3    R1,R2,R3          1K8
11       2    R7,R4             4K7
12       2    R5,R8             1K
13       3    R9,R12            150R
14       1    R6                680R
15       2    R11,R10           10R
16       1    R13               120R
17       2    U3,U1             H11L1 OPTO-ISOLATOR
18       1    U2                CNY17-3 OPTO-ISOLATOR
19       1    U4                MAX487, SN75176B
20       1    U5                LM7805

                                                                                                                                                                       2 July 2001