Q: What should I look for in an RS-485 converter to use with Windows?
A: A converter or interface card that has Automatic Send Data Control is strongly recommended for Windows and other multi-tasking Operating Systems. This is very important for half duplex 2-wire operation. Otherwise, data may be lost.
Q: What is the maximum connection distance using RS-485 devices?
A: The maximum distance of RS-485 without using a repeater is 4000 feet (1220 meters) at baud rates up to 90Kbps. This distance can be extended by adding an RS-485 Repeater or Optically Isolated Repeater every 4000 feet.
Q: Why do I need a RS-485 repeater for more than 32 nodes/devices?
A: A standard RS-485 node has a rated input impedance of 12Kohms. A standard RS-485 transmitter can't drive more than 32 devices and one pair of 120 ohm termination resistors. Simply adding an isolated repeater allows another 32 nodes to be connected.
Q: How many RS-485 devices can be on a single network?
A: Each 485 device must have a unique address. This typically allows for up to 256 nodes.
Q: Where can I use an RS-485 network?
A: Anywhere you need multiple devices at various locations and the devices have a built-in RS-485 port or can be adapted to RS-485 addressability. RS-485 has been used in many factory environments to gather data and control Addressable Motor Drives, Cameras, PLC's, Time Clocks, bar code scanners for process control and other factory equipment.
Q: What's the advantage of using RS-485 for data acquisition compared to RS-232?
A: The greatest advantage is noise immunity. Due to RS-485's differential signaling, it is much less affected by stray EMI/RFI "noise" than other serial protocols. Also, one serial port can talk to multiple devices at distances from a few feet to thousands of feet away. Additional serial ports aren't needed. Optically isolated converters can be used to protect the computer from voltages and ground loops from connected devices.
Q: What is the difference between RS-422 and RS-485?
A: RS-422 devices don't tri-state the transmitter, but RS-485 devices do. With RS-422, two units cannot connect to one receiver, because when one device tries to "talk" the other device is trying to hold the lines in one state, the other trying to toggle them high/low. RS-422 is always full duplex (if the device is not listen only), RS-485 can be full (4-wire mode) or half duplex (2-wire mode). RS-422 devices can have more signals for handshaking (RTS/CTS & DTR/DSR). RS-485 never has more than 2 signals, receive and transmit.