Answer: Since the RS-232-C interface standard was introduced earlier, it inevitably has shortcomings, mainly as follows: (1) The signal voltage levels of the interface are relatively high, which can easily damage the chips of the interface circuit. Additionally, due to incompatibility with TTL levels, a level conversion circuit is required to connect to TTL circuits. (2) The transmission rate is relatively low; in asynchronous transmission, the baud rate is 20Kbps. (3) The interface uses one signal line and one signal return line to form a common ground transmission, which is prone to common-mode interference, making it weak against noise interference. (4) The transmission distance is limited, with a maximum standard transmission distance of 50 feet, practically usable only up to about 50 meters. To address the shortcomings of RS-232-C, new interface standards have continuously emerged, and RS-485 is one of them, which has the following characteristics: 1. The electrical characteristics of RS-485: Logic ‘1’ is represented by a voltage difference of + (2—6) V between the two wires; logic ‘0’ is represented by a voltage difference of – (2—6) V. The signal voltage levels of the interface are lower than those of RS-232-C, making it less likely to damage the chips of the interface circuit, and this level is compatible with TTL levels, facilitating connection with TTL circuits. 2. The maximum data transmission rate of RS-485 is 10Mbps. 3. The RS-485 interface uses a combination of balanced drivers and differential receivers, enhancing its common-mode rejection capability, thus providing good noise immunity. 4. The maximum standard transmission distance of the RS-485 interface is 4000 feet, practically up to 3000 meters. Additionally, the RS-232-C interface allows only one transceiver on the bus, meaning it has single-station capability, while the RS-485 interface allows up to 128 transceivers on the bus, thus having multi-station capability. This allows users to easily establish a device network using a single RS-485 interface. Due to the excellent noise immunity, long transmission distance, and multi-station capabilities, RS-485 has become the preferred serial interface. Since the RS-485 interface forms a half-duplex network, generally only two wires are needed, so RS-485 interfaces use shielded twisted pairs for transmission. The RS-485 connector uses a DB-9 9-pin plug/socket, with the intelligent terminal RS-485 interface using a DB-9 (socket), and the keyboard interface connecting to the keyboard using a DB-9 (pin). When using the RS-485 interface, how should the length of the transmission cable be considered? Answer: When using the RS-485 interface, the maximum cable length allowed for data signal transmission from the generator to the load depends on the data signal rate. This length is mainly limited by signal distortion and noise. The curve shown below illustrates the relationship between maximum cable length and signal rate, using 24AWG copper core twisted pair telephone cable (0.51mm diameter), with inter-wire bypass capacitance of 52.5PF/M and terminal load resistance of 100 ohms. (The curve is referenced from GB11014-89 Appendix A). From the graph, it can be seen that when the data signal rate drops below 90Kbit/S, assuming the maximum allowable signal loss is 6dBV, the cable length is limited to 1200M. In practice, the curve shown is very conservative, and it is entirely possible to achieve greater cable lengths. When using cables of different diameters, the maximum cable lengths obtained will vary. For example: when the data signal rate is 600Kbit/S using 24AWG cable, the maximum cable length is 200m. If using 19AWG cable (0.91mm diameter), the cable length can exceed 200m; if using 28AWG cable (0.32mm diameter), the cable length can only be less than 200m.