This document summarizes a wireless mesh network system using XBee868LP modules. The system consists of two wireless I/O modules with 12 digital inputs and outputs each. The modules transmit input signals wirelessly to each other using mesh networking. This allows the addition of intermediate repeater modules to increase range or bypass obstacles. The system provides reliable and bidirectional communication between I/O modules in less than 100ms typically. It also implements fault detection mechanisms to monitor component health.

1. Mesh networks based on XBee868LP modules
Introduction
The proposed RF I/O modules are based on XBee868LP devices from Digi.
This radio implements Listen Before Talk (LBT) and Automatic Frequency Agility (AFA). This
virtually eliminates interference by listening to the radio environment before any transmission starts, and
automatically shifting to a new channel when interference is detected.
The XBee868LP modules are configured to operate in the DigiMesh networking mode. It allows
messages to be routed through several different nodes to a final destination. Self-healing attributes of
mesh networking are also allowed. In the event that one RF connection between nodes is lost (due to
power-loss, environmental obstructions, etc.) critical data can still reach its destination due to the mesh
networking capabilities embedded inside the modules.
No hierarchy and no parent-child relationships are needed. Reliable delivery of data is
accomplished by means of retries and acknowledgments.
RF performance
 License-free 863MHz to 870MHz frequency band in Europe (available up to 30 channels)
 LBT + AFA (Listen Before Talk and Adaptive Frequency Agility)
 RF Data Rate 80kbps
 Line-Of-Sight Range up to 4km w/2.1dBi antenna
 Indoor/Urban Range up to 150 m w/2.1 dBi antenna
 Transmit Power up to 14 dBm (25 mW) E.I.R.P. w/ 2.1dBi antenna, software selectable
 Receiver Sensitivity -101 dBm
 Antenna Options U.FL
 DigiMesh networks are defined with a unique network identifier. The ID parameter allows
multiple DigiMesh networks to co-exist on the same physical channel.
Functional features
This document briefly introduces a bi-directional wireless system designed for applications that
require remote control and cable replacement. Main advantages are as follows:
 Each I/O module comprises of 12 digital inputs (optocouplers) and 12 digital outputs (change
over relays)
 Galvanically isolated power supply, inputs and outputs
 Programmable delay to filter out bouncing of the input signals
 Fast bi-directional data transmission between two ultimate I/O modules with optional
intermediate repeater(s) to increase range or bypass various obstacles.
 Embedded mesh networking enables to enlarge operation range as much as required
 Data transfer time is typically less than 100ms without repeater(s).
 RS485 port with protection against 230VAC. It enables configuration of the I/O modules and
cable replacement applications. The XBee868LP modules allow remote (wireless) configuration
commands to manage devices in the network.
 Industrial temperature range -40 … +85o
C

2.  Wide supply voltage range +18 … +36VDC. Protection against 230VAC
 Max power consumption 4W (when all relays are turned on)
 Common Fault Relay to signal possible failures. The following fault signals are implemented to
improve reliability:
a) RF connection fault between ultimate I/O modules,
b) Configuration fault,
c) Power fault.
 Status LEDs to distinguish fault signals
Operation principles
The system consists of two wireless I/O modules (see Fig. 1). Each I/O module includes 12
digital inputs (optocouplers) and 12 digital outputs (relays). For this particular wireless system, outputs
of the Module B display inputs of the Module A. And outputs of the Module A display inputs of the
Module B. Arrows depict signal flow.
Fig. 1 Bidirectional remote control
An I/O module transmits input signals to its destination node after they change their state. This
approach enables to minimize delivery delays and avoid frequent periodical RF data transmission in
both directions. If input signals remain unchanged for a long time, the current state of the input signals is
transmitted each 10s to check connection with the destination I/O module. Delivery delay is usually less
than 100ms that is admissible for remote control applications.
Programmable delay for input signals
It is sometimes desirable to eliminate bouncing of the input signals. For this purpose, a
programmable delay is introduced for all inputs. Of course, addition of such a delay increases overall
delivery time respectively.
Configuration parameters
There are some configuration data like ID parameter, MAC address of the corresponding end
device and programmable delay time, which are stored in a non-volatile memory of the I/O modules.
Thus, the configured parameters endure loss of supply voltage. I/O modules always inspect correctness
of all configuration parameters and set the Configuration Fault signal when an error infiltrates.
Fault signals and Fault Relay
Each I/O wireless module includes Fault Relay (Relay 12, in particular) that signals fault
conditions, such as Power Fault, Configuration Fault and RF Connection Fault. In this way, the system
permanently controls its correct functionality and early informs about different fault conditions.

3. An I/O module detects Power Fault if output voltage of the isolated DC/DC converter drops
below +21V for some reason. Normally, it’s equal to +24V. Some unexpected conditions like short
circuit, overload, excessive temperature and other factors may result in voltage decrease. It is important
to monitor the output voltage because relays need a certain minimal voltage to operate properly.
When RF connection between ultimate I/O modules is lost and no data transfer occurs, RF
Connection Fault emerges.
The system discovers Configuration Fault when an error appears during reading of configuration
parameters from a non-volatile memory. This fault demands reconfiguration of all previously written
parameters to continue further operation.
When the connection between two ultimate I/O modules is lost for some reason, each module
periodically tries to establish it again. Fault Relay (Relay 12) will resume its normal state as soon as no
faults are present any more. All signal relays (Relays 11 … 1) are turned off, while no reception takes
place.
In addition, status LEDs distinguish fault signals.
Intermediate repeater(s)
Some applications may demand to place one or more repeaters between I/O modules. Embedded
DigiMesh networking permits to use repeaters without additional reconfiguration of the ultimate I/O
modules. That is a great advantage in comparison with other competing systems. Figure 2 shows
addition of an intermediate node (repeater) to bypass an obstacle.
Fig. 2 A system with one repeater
Additional repeaters may slightly increase delivery delays. A dedicated repeater is also available.
It doesn’t consist of any inputs and outputs. Therefore, it is smaller and cheaper.
In conclusion it is worthy to mention that the proposed mesh network affords to expand
operation area as much as necessary without exceeding of the maximal allowed transmission power.