USB to Serial Converters Manual
Version 2.1
Document Reference No.: CP_000032 Clearance No.: CP#022
Copyright © Connective Peripherals Pte Ltd
12
4
Switch and Jumper Settings
This section applies to ES-U-2xxx-x and ES-U-3xxx-x converters, which can be configured for different
interface types. These converters have DIP switches and internal jumpers to select the required interface.
The following pages give details of the settings required for each model of converter. Ensure that you
refer to the correct section for your model of converter. The relevant model numbers are listed at the top
of each section. Always read the general notes and warnings on this page before proceeding to configure
your device.
4.1
General Notes and Warnings
Safety
For ES-U-xxxx-RM models with internal power supplies, the power cable must be disconnected
before removing the cover. Opening the cover will expose connections and boards carrying lethal
voltages. Contact with these will cause severe injury or death. In common with all switching
power supplies connected to the mains (120/240VAC), the power supply board in these products
can remain charged with dangerous voltages for some time after the power input is disconnected
and so avoid contact with the power supply board inside the converter even when un-powered.
Serial Bus Configuration
It is important to configure the DIP switches and jumpers to select the correct type of serial bus (RS-232,
RS-422 or RS-485) for your application before connecting the ES-U-xxxx-x converter to your serial bus.
Otherwise, damage can occur to the converter or other equipment on the RS-232/RS-422/RS-485 bus.
To avoid damage, the power supply (or power cable in rack-mount units), 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 not needed for RS232 and may affect the signals if enabled.
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.
The ES-U-xxxx-x converters have jumpers (or DIP switches) to allow internal termination resistors to be
enabled if required. This avoids the need to provide an external resistor at the ES-U-xxxx-x end of the
bus.
ES-U-xxxx-x converters have separate jumpers/switches for Tx+/Tx- termination and Rx+/Rx-
termination allowing termination to be enabled for either or both if required. The product-specific sections
throughout chapter 4 describe these in more detail.
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.
Some ES-U-xxxx-x models feature internal 750-ohm resistors with jumpers to enable/disable them.
