Six Questions On RS485 Standard

A second installation has two desktop PCs sitting very close to each other. This installation should have a third wire connecting the earth ground/circuit common from the desktop PC to the laptop's RS-485 port circuit common. The RS-485 circuit common is connected to earth ground in both PCs. This part mainly discusses some common sense. The first is to connect the signal common of the driver/receiver circuit to "protective or frame ground" through a 100 Ω resistor. RS-422 voltages are referenced differentially from one wire to the other, but are also referenced to circuit common. 3V (referenced to the signal common wire). This will reduce the reflections on a RS-422 network to some extent, but RS-422 also recommends a termination resistor at the receiver's end of the wires (well it sort of recommends, depending on the "data rate" or the "signal rise time at the load end of the cable"). Some manufacturers get around this problem by setting their receivers input threshold to a slightly biased value, such as -50mV, instead of exactly at 0V. When the wire pair is not driven, the receiver will see the input as an idle condition (binary 1). If the transmitter is turned on at the exact same time as the edge of the start bit, the receiver will see the voltage change from idle to start so the UART will always see the first start bit transmitted.

For this application to work, the driver must transmit an idle (binary 1) after the driver is enabled, for some period of time before the start bit is transmitted. The first bit a UART transmits is the start bit (0) and the receiving UART should see this transition. RS-485 requires the driver and receiver to function if the common mode voltage is shifted against circuit common (see the section on voltages for more information). This limited the open circuit voltage to ±12V. The driver must output between 5V and 15V in magnitude into a load of 3kΩ to 7kΩ. The driver must not be able to output more than 100mA when shorted to any other conductor in the cable, must not be able to output more than 25V and must be able to handle an open circuit, or a short to any other conductor in the cable. Lower voltage drivers are now available to allow operation from battery powered devices and the open circuit voltages may be lower than ±12V. The second optional grounding arrangement is to connect the circuit common directly to the frame ground without a resistor. It shows a diagram of the driver and receiver with two wires connecting them, and a third point "C" that is called a common.

There is no wire shown connecting this third point between driver and receiver. 12V and -7V. If the desktop's port is ESD protected damage may not occur, but there is no guarantee. Since the annex admits that the installation can cause physical damage to components, it is not surprising that grounding is a contentious subject. If the circuit common is truly isolated from earth ground, then scuffing your feet on carpet (to pick up an ESD charge) and touching the wires can cause the wires to shift potential several thousand volts. When 5V drivers are used, the driver typically pulls one wire to circuit common, and the other wire to 5V (and vice-verse for the opposite data). RS-422 has one or more pair of wires. When a binary 1 is transmitted, the signal is spacing, RS485 standard and the voltage on the wire must be more negative than -3V. In most digital logic a binary 0 is usually considered OFF and a binary 1 is ON. 7V. The receiver must recognize a differential voltage of greater than ±200mV as a binary value. The receiver is designed to operate with voltages between 3 and 15V in magnitude (i.e. both positive and negative voltages) but must be able to handle an input of 25V without damage.

This means that low data rates can handle an improperly terminated (or even unterminated) cable better than high data rates. Even if you use 1% resistors, the standard values would be 127Ω and 1.47kΩ. This would result in a total termination resistance of 121.7Ω and a bias voltage of 0.207V. Since the tolerance of the resistors means that you will never get the ideal calculated values, don't sweat it. Service of standard protocols (e.g. MODBUS, SNMP), possibility of installation of dedicated user protocols. The annex also says that certain applications may cause the resistor to fail so the installation must allow access for inspection and replacement. The following example is to bias the network to 200mV. You may want to bias the network to a higher differential voltage to improve the noise margin. RS-422 is similar (except they leave the hyphen out) Both standards have a section defining why you may want fail-safe operation, but neither discuss how to implement it. The fastest RS-485 transceivers out there right now are running about 50 megabits. The ringing occurs when the data transitions, and will eventually damp out.