Baldor NextMove ST, Installation Manual

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Input / Output 4-19 MN1929
4.6.4 Multidrop using RS485 / RS422
Multidrop systems allow one device to act as a ‘network master’, controlling and interacting
with the other (slave) devices on the network. The network master can be a controller such as
NextMove ST, a host application such as WorkBench v5 (or other custom application), or a
programmable logic controller (PLC). RS422 may be used for multi-drop applications as
shown in Figure 20. Four-wire RS485 may be used for single point-to-point applications
involving only one Baldor controller. If firmware is updated over RS485/RS422, it can only be
downloaded to the controller that was chosen in the Select Controller dialog in WorkBench v5.
Master and final slave are
shown with terminating
resistors, T
R
, typical value
120 Ω .
Jumper JP2 connects an
internal 120 Ω terminating
resistor. JP2 is located just
behind the serial connector on
the NextMove ST control card.
Network slave
T
R
RX-
DGND
RX+
TX+
TX- RX-
DGND
RX+
TX+
TX-
RX-
DGND
RX+
TX+
TX-
Twisted pairs
T
R
Connect overall shield
to connector backshell.
Network slave
Network
master
Figure 20 - 4-wire RS422 multi-drop connections
Each transmit/receive (TX/RX) network requires a termination resistor at the final RX
connection, but intermediate devices must not be fitted with termination resistors. An exception
is where repeaters are being used which may correctly contain termination resistors.
Termination resistors are used to match the impedance of the load to the impedance of the
transmission line (cable) being used. Unmatched impedance causes the transmitted signal to
not be fully absorbed by the load. This causes a portion of the signal to be reflected back into
the transmission line as noise. If the source impedance, transmission line impedance, and
load impedance are all equal, the reflections (noise) are eliminated. Termination resistors
increase the load current and sometimes change the bias requirements and increase the
complexity of the system.