Basics of the RS-485 Standard
What is an RS-485 network?
How does the hardware work?
An RS-485 network can be configured in two ways: “two-wire” or “four- wire.” In a “two-wire” network the transmitter and receiver of each device are connected to a twisted pair. “Four-wire” networks have one master port with the transmitter connected to each of the “slave” receivers on one twisted pair. The “slave” transmitters are all connected to the “master” receiver on a second twisted pair.
In either configuration, devices are addressable, allowing each node to be communicated to independently. Only one device can drive the line at a time, so drivers must be put into a high-impedance mode (tri-state) when they are not in use. Some RS-485 hardware handles this automatically. In other cases, the RS-485 device software must use a control line to handle the driver. (If your RS-485 device is controlled through an RS-232 serial port, this is typically done with the RTS handshake line.)
A consequence of tri-stating the drivers is a delay between the end of a transmission and when the driver is tri-stated. This turn-around delay is an important part of a two-wire network because, during that time, no other transmissions can occur (not the case in a four-wire configuration). An ideal delay is the length of one character at the current baud rate (i.e. 1 ms at 9600 baud). The device’s manufacturer should be able to supply information on the delay for their products.
Two-wire or four-wire?
Four-wire networks, on the other hand, allow full-duplex
Remember when ordering your cable, “two-wire” is really two wires + ground, and “four-wire” is really four wires + ground.
How does the software work?
Connecting a multidrop 485 network.
Signal Ground – don’t forget it.
Over a distance of hundreds or thousands of meters/feet, there can be very significant differences in the voltage level of “ground.” RS-485 networks can typically maintain correct data with a difference of -7 to +12 Volts. If the grounds differ more than that amount, data will be lost and often the port itself will be damaged.
The function of the signal ground wire is to tie the signal ground of each of the nodes to one common ground. However, if the differences in signal grounds