Specifications

Appendix C - Glossary of Terms RS-422 And RS-485 Applications Ebook
Page 121 of 137 Manual Documentation Number: <DocNumber>
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RS-485 systems can be half duplex two-wire systems (one twisted pair plus
signal common/ground) or full duplex four-wire systems. An RS-485
transmitter driver is activated to send data and is set to a high impedance tri-
state at the end of transmission. Driver control can be automatic using a Send
Data circuit, or manual by setting the RTS line or UART RTS control high for
transmit, then low at the end of transmission. In a half duplex two-wire
system, the receiver is set to receive except when transmitting.
In a two-wire system, all slaves and masters are normally in the receive mode.
When one master transmits, all slaves and masters receive the signal and
response, and all slaves must be able to ignore commands and responses
to/from other slaves. Each slave must wait until transmit is finished plus a
delay (for bus turn-around), before responding.
In a four-wire system, all slaves are connected to the transmitter of the
master(s). All slaves connect to the receiver of the master(s). Each slave must
respond only to commands addressed to it, but no turn-around delay is
needed. The slave can start responding immediately, even while receiving.
Other slaves never hear each other's responses.
RS-485 Connector
This is the connector used on the RS-485 side of the converter or device.
RS-485 Fail-Safe Port Isolator
A device intended for use in a RS-485 network to isolate a node which has
become stuck in a MARK or SPACE state or which has lost power and is loading
down the rest of the 485 network so other devices cannot be accessed. When a
node stops working, the port isolator disconnects it from the remainder of the
network.
RS-485 Network
The RS-485 wiring and slave devices interconnected to one or more masters.
From a hardware point of view the number of devices on a RS-485 network is
restricted to 32 nodes unless repeaters are used. From a software point of
view the number of nodes is only restricted by the addressing scheme of the
devices, typically 255 for a single byte address.