 RS-485 Connections FAQ
How Do I Make RS-485 Connections?
First check
the pinout connections and specifications of the devices you
want to connect in order to determine whether you need a "2-wire"
RS-485 connection or a "4-wire" RS-485 connection. If you
can't obtain this information before selecting a converter
or serial card, select a converter or device which can be
easily set to work in either mode using switches, jumpers
or other options. Most often you will be setting up and connecting
a card or converter as a Master to one or more Slave devices.
Following are figures for three types of 2-wire Master connections,
and one type of 4-wire connection. Usually Termination is
not required for baud rates 19.2K or lower at distances up
to 4000 ft.(1220 m), so it is not shown in the simplified
figures which follow. (see note on page 3 regarding markings
and pinouts)
Figure 1 applies to many Ethernet/Serial
Servers and a few B&B RS-485 Converters such as the 485SD9TB,
485LPRJ, 485SD9R, 485SD9RJ.
Figure 2 applies to several B&B Converter
models such as the 485DRC, 485LDRC, 485LDRC9, 489OT9L, 485OTLED
and 4WSD9R. Two switches are set for 2W or to the 2-Wire RS-485
Mode.
Figure 3 applies to most B&B RS-485 Converters
or Serial Cards that can be set for 2-wire or 4-wire operation,
and for some 2-wire Converters that use the same circuit board
for the RS-422 model. Check the Data Sheet schematic or block
diagram. Figure 3 is also used for Repeaters/Optical Isolated
Repeaters such as the 485RPTR, 485OP, 485OPDR. A RS-422 device
does not tri-state the transmitter so it cannot use these
connections.
Some Converter models are: 485BAT3, 485COR,
485CON, 485COSR, 485COSN, 485CSP2, 485TBLED, 485LP9N, 485LP9,
P485BTB1, 485HSPR.
4-Wire Connections
This connection is used for a single
Master in a 4-wire system. When only one master is used, the
Master can be set to RS-422 mode since the there is no need
to share the transmit wire pair to the Receivers of all the
devices. All of the devices must be RS-485 and Tri-state since
the same wire pair to the Receiver must be shared by the Transmitters
of all the devices. The advantage of the 4-wire connection
is that all devices only see commands from the Master, and
no Device sees the responses from other devices. Turn-around
delay is also not required.
Multi-Master Connections
2-Wire Multi-Master
Connect additional Masters the same as
any Device, (Figs. 1-3) matching the polarity of the A and
B data lines. All devices must be able to ignore the commands
to other devices and able to ignore the responses from other
devices. Any device could transmit to all others if permitted
or some special Protocol is established to avoid data collisions.
Usually the devices are Slave type and respond only to requests
from a Master Device.
4-Wire Multi-Master
This connection is less common, Master
2 may be anywhere else on the network and may be used for
collecting data returned from the devices. It can monitor
the data from the devices only, but is not able to monitor
the data requests from the other Master without additional
connections. Only one Master can talk to the Devices at a
time, so one Master must stop transmitting for a time (to
avoid data collisions) so the other Master can access the
same devices, or use some software protocol established for
Device sharing. When Master 2 is used as Backup, the program
on Master 2 begins polling the devices when the slave Devices
stop responding for a predetermined time.
Note on RS-485 Pinout markings: Sometimes
a device will be marked with the polarity reversed from normal
RS-485 standards where the Data A or TD(A)/RD(A) line is (+)
and the Data B line or TD(B)/RD(B) is (-). In a system with
2-wire connections, try reversing the lines to see if the
device will respond. In a system with 4-wire connections,
try reversing the receive pair polarity and transmit pair
polarity. If the connections are wrong, you won't be able
to communicate because the data bits are upside down, but
swapping polarity won't damage anything. The marking Rx or
Tx may be used instead of TD and RD or in a 2-wire system,
TxRx+ and TxRx-, or Dx+ and Dx- for the Data lines. Many naming
schemes are in use. RS-485 pinouts also have many variations,
there is no single standard pinout arrangement or connector
type, DB9, DB15, DB25, RJ-11, RJ-12, RJ-45 or DIN connector.
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