RS485协议(PDF)

RS-485 Driver Control The principle difference between RS-422 and RS-485 is that the RS-485 driver can be put into a high impedance, tristate mode, which allows other drivers to transmit over the same pair of wires. There are two methods of tristating an RS-485 driver. The first method is to use a control line, often the RTS hands hake line, to enable and disable the driver. This requires that the host software raise the RTS line before beginning a transmission to enable the driver, then lower the RTS line after the completion of the transmission. Since only a single RS-485 driver can be enabled on a network at one time it is important that the driver is disabled as quickly as possible after transmission to avoid two drivers trying to control the lines simultaneously, a condition called line contention. Under some operating systems it can be difficult to lower RTS in a timely manner and this method of driver control should be avoided altogether. hake line, to enable and disable the driver. This requires that the host software raise the RTS line before beginning a transmission to enable the driver, then lower the RTS line after the completion of the transmission. Since only a single RS-485 driver can be enabled on a network at one time it is important that the driver is disabled as quickly as possible after transmission to avoid two drivers trying to control the lines simultaneously, a condition called line contention. Under some operating systems it can be difficult to lower RTS in a timely manner and this method of driver control should be avoided altogether.