Specifications

Appendix C - Glossary of Terms RS-422 And RS-485 Applications Ebook
Page 99 of 137 Manual Documentation Number: <DocNumber>
www.bb-elec.com/
www.bb-europe.com/
Glossary of Terms
Symbols & Numeric
485 Driver Control
RS-485 transmitters must be enabled to transmit or disabled when not
transmitting. When disabled, the transmitter output presents a high impedance
to the transmission line (tri-stated). The 485 driver must be controlled unless
set to always enabled for RS-422 operation.
Depending on the device circuitry and model, the driver control may be RTS
controlled or SD (Automatic Send Data) controlled.
RTS controlled drivers require special software to raise and lower the RTS line
on the RS-232 port for each transmission. RTS control is not usually suitable
for Windows systems because RTS timing cannot be closely controlled, which
can result in lost data.
In SD mode, additional circuitry inside the device controls the transmitter.
Some models have a preset fixed R/C timing circuit preset for 9600, or switch
selectable presets, or bit-wise enable timing. Serial cards may use bit counters
or the UART Transmit buffer status line for control.
In Half Duplex operation, the 485 Driver Control signal is also used to disable
receive during transmit, and enable it at the end of transmit.
485 Wires
Two-wire RS-485 system is half duplex using one twisted wire pair plus Signal
Ground/Common. The wire pair is used alternately for either transmit or
receive by the master or slaves.
Four-wire RS-485 system is full duplex using two twisted wire pairs plus Signal
Ground/Common. One pair is used for transmit from master(s) to slaves, the
second pair for the slaves to transmit back to the receiver on the master(s).
Software flow control can be used since both transmit and receive are active at
once.