September 2012
L010194
6
RS485 Protocol
The RS485 protocol is as follows, onboard receivers will remain in active mode indefinitely.
Transmitters must be turned off when the unit is not sending data to prevent the line from send-
ing and receiving data at the same time. Therefore when the PC is transmitting data its driver
will be turned on and each of the units connected will have their drivers off. If they are requested
to send data back to the PC, the selected unit will turn it’s driver on to send the data then turn it
off after it has completed transmission. Note: The above protocol is done internally between the
converter and controller. The RS485 method of communication allows increased noise immunity
and increased communication distance of up to 4000 feet without repeaters. RS485 repeaters
allow an additional 4000 feet per repeater.
Two Wire Configuration
The two wire configuration reduces cabling costs by requiring only three wires. A, B and ground.
The DPC50501 is designed to allow either the two or four wire configuration. To use the 2 wire
configuration simply wire TX+ to RX+, and TX- to RX- on your converter box. Then run a wire
from ground, a wire from TX+/RX+ and a wire from TX-/RX-, to the first controller in the network.
Finally do the same on the terminal block of the controller to the converter box. The diagram below
illustrates how this configuration is connected. RS422 systems require a dedicated pair of wires
for each signal, a transmit pair, a receive pair, and an additional pair for each handshake/control
signal used (if required). The tristate capabilities of RS485 allow a single pair of wires to share
transmit and receive signals for half duplex communications. This “two wire” configuration (note
that an additional ground conductor should be used) reduces cabling cost. RS485 devices may
be internally or externally configured for two wire systems. Internally configured RS485 devices
simply provide “A” and “B” connections (sometimes labeled “-” and “+”).
Four Wire Configuration
Devices configured for four wire communications use TX and RX connections for both the transmit
and the receive pairs. The user can connect the transmit lines to the receive lines to create a
two wire configuration flexibility. Note that the signal ground line should also be connected in the
system. This connection is necessary to keep the VCM common mode voltage at the receiver
within a safe range. From the diagram below, it can be seen that all wires are run directly from
the converter to the controller. For example TX+ from the converter goes to TX+ on the control
-
ler and so on.
July 2018