ES-U-2001-S
Installation Guide
Version 1.1
Document Reference No.: CP_000074 Clearance No.: CP#062
Copyright © Connective Peripherals Pte Ltd
5
3
Hardware Setup
Before connecting the unit to the RS422/RS485 bus or to the computers USB port, you should set up
the hardware switches and jumpers.
3.1
General notes
Serial Bus Configuration
It is important to configure the DIP switches and jumpers to select the correct type of serial bus (RS-
422 or RS-485) for your application before connecting the converter to your serial bus. Otherwise,
damage can occur to the converter or other equipment on the RS-422/RS-485 bus.
To avoid damage, the serial cables and USB cable should be disconnected from your converter before
changing DIP switch settings and before opening the case to change jumper settings.
Termination
For RS-422 or RS-485, it may be necessary to enable termination of the data transmission lines. This
depends on the way in which your network is connected and if termination is already provided by
other devices on the network. Before applying the termination, check your cable specification for
proper impedance matching. The termination option connects a 120 Ohm resistor between the data
+ and - lines.
Termination is normally used in the node at each end of the bus, but nodes in the middle of the bus
should not have termination enabled. Enabling termination resistors on additional nodes which are
not at the ends of the bus may cause overloading of the line drivers leading to unreliable operation
of the unit and the overall system.
ES-U-2001-S converters have jumpers to allow the internal termination resistors to be enabled if
required. This avoids the need to provide an external resistor at the ES-U-2001-S end of the bus.
There are separate jumpers for Tx+/Tx- termination and Rx+/Rx- termination allowing termination
to be enabled for either or both if required. When using RS-485 half-duplex, the Tx and Rx pairs are
combined into a single bi-directional pair and so only the Tx+/Tx- termination should be enabled. The
Rx+/Rx- termination should be disabled.
Biasing
In some cases, the RS-485 lines may float to undefined levels when no transmitter is active, and this
can cause additional unexpected characters to appear at the start or end of a message. Biasing
resistors are used to set a defined logic level on the serial lines when no node is transmitting. 750
Ohm resistors pull up the Data + line and pull down the Data
–
line.
ES-U-2001-S converters feature internal biasing resistors with jumpers to enable/disable them.
These should be disabled if another device on your serial bus already provides biasing as biasing of
data lines must only occur at a single point in the cabling. Fitting more than one set of bias resistors
can cause overloading of the line drivers leading to unreliable operation of the unit and the overall
system.
There are four jumpers to select biasing on Tx+, Tx-, Rx+ and Rx-.
When using RS-485 half-duplex,
the Tx and Rx pairs are combined into a single bi-directional pair and so only the jumpers for Tx
biasing (
Pull up of Tx+
and
Pull down of Tx-
) should be fitted. The Rx biasing should be disabled.
