RS-485 tutorial
- an overview of the RS485 standard used for the
transmission of high speed serial data
RS485 is a standard for the transmission of serial data long
a hard wired cable. The system is defined under EIA/TIA-485 and RS-485 provides
the ability for multi-drop cabling and for speeds of up to 10 Mbps over short
runs of up to 50 feet and slower communications at speeds of 100 kbps at
distances of 4000 feet. Although not widely used in domestic environments, it is
widely used for data acquisition applications.
Applications
Often RS-485 links are used for simple networks, and they may be connected in a
2 or 4 wire mode. In a typical applications several address able devices may be
linked to a single controlled (PC), and in this way a single line may be used
for communication. It is also possible to convert between RS485 and RS232 using
simple interface converters that may include optical isolation between the two
circuits as well as surge suppression for any electrical 'spikes' that may be
picked up.
Using RS-485, it is possible to construct a multi-point data
communications network. The standard specifies that up to 32 drivers or
transmitters along with 32 receivers can be used on a system. This means that
there can be 32 nodes capable to both transmit and receive. This can be extended
further by using "automatic" repeaters and high-impedance drivers / receivers.
In this way it is possible to have hundreds of nodes on a network. In addition
to this, RS485 extends the common mode range for both drivers and receivers in
the "tri-state" mode and with power off. Also, RS-485 drivers are able to
withstand "data collisions" (bus contention) problems and bus fault conditions.
As RS-485 networks become larger, the problem of data
collisions becomes greater. This can be solved, at least in part by ensuring the
hardware units (converters, repeaters, micro-processor controls) are designed to
remain in a receive mode until they are ready to transmit data.
Another approach is to design a 'single master' system. Here
the master initiates a communications request to a "slave node" by addressing
that unit. The hardware detects the start-bit of the transmission and thereby
enables the transmitter. Once a requested data is sent the hardware reverts back
into a receive mode.
RS485 specification overview
| Attribute |
Specification |
| Cabling |
Multi-drop |
| Number of devices |
32 transmitters
32 receivers |
| Communications modes |
half duplex |
| Maximum distance |
4000 feet @ 100 kbps |
| Maximum data rate |
10 Mbps @ 50 feet |
| Signalling |
Balanced |
Mark (data = 1)
condition |
1.5 V to 5 V (B greater than A) |
Space (data = 0)
condition |
1.5 V to 5 V (A greater than B> |
| Driver output current capability |
250 mA |
|
