Solution:

COM Port Interface Pin-Assignment Data Flow Remark COM1,3,4 RS-232 1 DCD 6 DSR 2 RX 7 RTS 3 TX 8 CTS 4 DTR 9 RI 5 GND Full Duplex COM2 RS-232 1 DCD 6 DSR 2 RX 7 RTS 3 TX 8 CTS 4 DTR 9 RI 5 GND Full Duplex RS-232/422/485 is selectable by jumpers RS-422 1 TX- 6 DSR 2 TX+ 7 RTS 3 RX+ 8 CTS 4 RX- 9 RI 5 GND Full Duplex Driver: RTS control Receiver: Always on Pin-6, 7, 8, 9 are not suitable for RS-422 interface, please do not use them RS-485 1 DATA- 2 DATA+ 5 GND Half Duplex RTS control RTS register: Base+4 (MCR) Bit 1 Notice: 1. RTS control It is necessary to send messages via Driver and retrieve reponses via Receiver. However, under RS-422/485 interface, RTS ( Request To Send ) is often designed to control the action of Driver/Receiver, that means we must use RTS to enable Driver before sending messages. 2. How to use RTS control under RS-485 outportb((base+4),(inportb(base+4) | 2)); /* Set RTS high to enable Driver ( disable Receiver ) */ while (flag) { /* --- Sending data --- */ } outportb((base+4),(inportb(base+4) & 0x0fd)); /* Set RTS low to disable Driver ( enable Receiver ) */ while (flag) { /* --- Receiving data --- */ } 3. Automatic control Due to multi-thread, the timing problem of RTS signal must be existed under multi-task environments. The best way to avoid the phenomenon is to adopt automatic control ( such as ADAM-4520/4522 ), what you need to do is send and receive messages directly, without caring about RTS signal. Feedback: If you got any trouble during installation and can not solve it from above information, please c