Adding Wireless Instruments to Existing Networks |

Adding Wireless Instruments to Existing Networks

Adding Wireless Instruments to Existing Networks

By Moazzam Shamsi MSc, BEng, CEng, MinstMC; Global Solutions Architect with Emerson Process Management

The purpose of this article is to describe the steps involved in adding a WirelessHART instrument to an existing network. Commonly, users start out by deploying a WirelessHART network to monitor a specific process or asset. Often, users discover the need to add additional WirelessHART instruments to measure more process parameters.

The scalability of WirelessHART networks means that multiple process monitoring and diagnostics applications can reside on the same network because data segregation is performed at the system level. This means new instruments can be added to existing networks without any concern for how the data from the instruments will eventually be used, as these decisions are made at the system as opposed to the network level.

In a WirelessHART network, each instrument is connected to a WirelessHART gateway via a mesh network. Some instruments connect to the gateway directly, while others connect through other instruments, each of which can act as a repeater. These connections are dynamic and are made automatically through the wireless mesh network for both existing and new instruments.

New Instrument Installation Steps

Here are the steps for installing a new WirelessHART instrument to an existing network:

  1. Check the capacity of the gateway to verify whether it is capable of supporting the additional instrument(s) at the required scan rates. Figure 1 is a screenshot of an online tool which can be used to perform this step.

    Figure 1. This online tool can be used to estimate WirelessHART gateway capacity, the first step when adding a new instrument to an existing network.  

  2. Follow these basic Network Planning rules. Rules a. and b. are included for guidance only as these two rules would have been followed when originally implementing the network.
    1. Each network should be scoped to a single process unit
    3. Each network must contain a minimum of five wireless instruments within effective range of the gateway
    5. Have 25% of the wireless instruments in the network within range of the gateway
    7. Each wireless instrument must have threeneighboringinstruments within effective range
    9. Effective range is determined by the type of process unit with respect to the density of infrastructure which may cause obstruction of wireless signals
  3. Refer to the WirelessHART System Engineering guide for detailed planning rules (Reference 1). Add the prospective instruments to the WirelessHART Planning tool (Reference 2) and verify if the additional instruments meet the WirelessHART rules.
  5. When adding a new instrument to the network, turn on active advertising in the gateway as this will improve the join time. The join time is the amount of time required for a new instrument to be recognized by the gateway and join the network.
  7. When adding and commissioning new instruments, start with instruments closest to the gateway and expand out as instruments farther out will use instruments closest to the gateway to connect.
  9. Each new instrument needs a Network ID and Join key, both of which can be obtained from the network systems administrator. This ensures security as no new instrument or device can be added to the network without the permission of the systems administrator.
  11. The following are the most common ways of adding an instrument to a network:
    1. HART Field communicator: Connect the communicator to the instrument and enter the Network ID and Join Key (Figure 2). The instrument will then automatically join the network.

      Figure 2. A handheld HART field communicator can be used to add WirelessHART instruments to an existing network.

    2. Asset Management System (AMS) Instrument Configurator running on a PC (more efficient when adding multiple instruments to a network):
      • Connect the instrument to the AMS via a HART Modem using the PC’s USB port
      • The instrument should appear under the HART modem tree
      • Drag and drop the instrument on the gateway it will join (Figure 3). The network ID and Join key will automatically be transferred to the instrument.

        Figure 3. Emerson AMS software application is more efficient than a field communicator when adding multiple instruments to an existing wireless network.

  12. Instruments can also be factory pre-configured with a Network ID and Join Key. This is common in new installations, but with existing installations the system administrator usually provides this configuration information.
  14. After an instrument has joined the network its join status can be checked at the gateway or at the AMS instrument dashboard screen (Figure 4). The join status can also be checked via a handheld HART Field communicator.

    Figure 4. The status of a new wireless instrument added to an existing network can be checked at the gateway, or the device dashboard depicted in this screen shot.

  16. Calibrations and sensor trims for instruments such as pressure and temperature are generally the same as for wired instruments, but check the instrument manual for instrument specific setup options.

Connecting Instruments to the Host

Each instrument in the network must be connected to a host system through a gateway. Examples of host systems are distributed control systems, asset management systems, process data historians and other process plant control and monitoring systems.

The gateway is hardwired to the host through a digital communications link such as Modbus RTU, Modbus TCP or HART IP. The gateway converts data from the WirelessHART field network into the desired protocol and physical layer needed for integration into the host system. The gateway performs all management of the WirelessHART network and manages communications to and from the WirelessHART instruments. The host system therefore does not require any special software to support the WirelessHART field network.

Integration of data originating from a gateway into a host control system is normally performed in one of two ways: through native connectivity directly to the host system, or using standard protocols such as Modbus or OPC. For native connectivity including vendor specific I/O cards, contact the host vendor.

OPC and Modbus are non-proprietary protocols and use standard data exchange and integration techniques to map data from the gateway into the host control system. Typical data mapped to the host are process variables (PV, SV, TV, QV), time stamps (if using OPC) and overall device status. Diagnostic information is typically passed to an asset management system via Ethernet.

WirelessHART devices provide internal diagnostics and process variables like any wired HART device. The gateway also provides additional diagnostics regarding network performance. Data from WirelessHART instruments will not propagate to the host system if the data is deemed questionable by the gateway based on either a HART diagnostic message or due to an extended delay in reception at the gateway from the WirelessHART instrument.

The gateway can notify the host system if communication problems exist. Additionally, the gateway is responsible for WirelessHART network management and network diagnostics.

The extent of diagnostics between the gateway and the host system will depend on the host system capabilities.

Multiple Host Systems

Often, existing host systems can be a combination of legacy DCS and PLC platforms, and modern data management solutions such as process data historians. WirelessHART gateways support multiple connections into multiple host systems over multiple protocols for these types of applications.

This enables WirelessHART networks to support modernization of an existing host system. For example, suppose the existing DCS has no spare capacity for 4-20 mA signals from wired HART instruments. A WirelessHART network could be connected to the DCS to eliminate the need for moreanaloginput cards. In parallel, HART diagnostic data could flow to an asset management program from existing wired HART devices equipped with WirelessHART adapters.

This type of modernization project could enable incremental modernization with an older host system. When a scheduled plant turnaround occurs to upgrade the DCS, the existing WirelessHART networks would transition to the new host system.

Following these steps will allow users to quickly add WirelessHART instruments to an existing network. If capacity is available on the network, new instruments can be added quickly and with minimal expense. If capacity is not available, a new gateway can be added to increase network capacity. See the articles listed below in the Conclusion and the References for gateway loading and other information.


This is the fifth of a five-article series on wireless instrumentation and infrastructure. This article showed how to add instruments to an existing WirelessHART network. For more detail on these and other subjects listed below, please see Reference 1.

The first article showed how wireless can be used to cut operating expenses in capital-constrained environments.

The second article covered wireless instrument system planning, design, test and commissioning.

The third article covered WirelessHART field instrument installation and configuration.

The fourth article covered showed how using wired and wireless field instrument networks simultaneously increases reliability and reduces required maintenance.

Now that this five-article series is concluded, the reader will be prepared to justify, design, install and maintain wireless instrumentation and wireless networks.

About the Author

Mr. Shamsi has been an automation professional for 25 years and his career spans a broad range of industries, in roles from technical leader to project management. He presently works for Emerson Process Management where he directs Emerson’s global wireless consulting and execution solutions on large capital projects. He specializes in working with clients and contractors to implement technology solutions to improve operational efficiency.