Technical information

What To Do First Serial Solutions
differential data transmission system. Over distances up to 40
feet the maximum data rate is 10 Megabits per second, and for
distances up to 4000 feet the maximum data rate is 100
Kilobytes per second, allowing communication much further and
faster than RS232. RS485 allows up to 32 transmitter receiver
pairs to be present on the line at one time. If more than one
device may transmit data, the PC’s RTS line is used as as
transmit enable signal, so preventing contention between talkers.
RS485 signal lines.
TXD, Transmitted data an output from PC Data line.
RXD, Received data an input to PC. Data line.
RS485 ports only have a transmitted data line pair, a
received data line pair. Frequently, to save having too many
wires, the RS485 system shares the transmitted and received data
lines.
The RS485 bus is implemented in one of three ways and
the handshaking option is set accordingly. These three RS485
implementations are listen only devices, party line half duplex
and party line full duplex.
RS485: PC To Listen Only Devices. One cable.
The RS485 listen only device handshake connects one PC
to one or mode devices using only ONE twisted pair cable. The
transmit lines of the PC are connected to the receive inputs of
the devices. The external RS485 devices can only listen to data
received from the PC and do not ever reply back to the PC. The
PC need only ever transmit data. It does not receive any data.
Thus the RS485 card is in transmit only mode, this is handshake
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