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H1 FOUNDATION Fieldbus Data Link Layer

Layer 2 of the OSI Reference Model is where we define the “data link” elements of a digital data network. The H1 FF network exhibits the following properties:

Master/slave network behavior for cyclic communications (i.e. one device polls the others, and the others merely respond)

Delegated token network behavior for acyclic communications (i.e. devices serially granted time to broadcast at will)

Dedicated “scheduler” device for coordinating all segment communications

8-bit address field (0 through 255 possible)

Maximum of 32 “live” devices on a segment

On an operating H1 segment, one device called the Link Active Scheduler (abbreviated LAS) functions as the “master” device for coordinating all network communications. For time-critical transmissions, the LAS polls the various field instruments to transmit their process control data (process variables, PID control output values, and other variables essential for loop monitoring and control), while these other devices respond in answer to the LAS’s queries. These critical communications occur on a regular schedule, and therefore are referred to as scheduled or cyclic communications. Cyclic communication operates in a “master-slave” fashion, with the LAS acting as the master (commanding slave devices to broadcast their critical data), and all other devices responding only when called upon by the LAS.

Periods of time in between these critical transmissions are used for device’s internal processing (e.g. PID algorithm execution, diagnostic checking) and also for less-critical data transmission. It is during these unscheduled or acyclic times that devices are sequentially given permission by the LAS to broadcast data of less importance such as operator setpoints, PID tuning constant updates, alarm acknowledgments, and diagnostic messages. Acyclic communication operates in a manner similar to “token-passing,” with the LAS issuing time-limited tokens to the other devices in sequence permitting them to freely broadcast whatever other data they have to share. The scheduled nature of cyclic communication guarantees a certain maximum response time to critical control functions, an important property of control networks called determinism. Without determinism, a control system cannot be relied upon to perform critical regulatory functions in a timely1 manner, and sequencing2 of control functions such as PID, summers, subtractors, ratio multipliers, and the like may be compromised.

Device addressing

FOUNDATION Fieldbus devices (also called nodes) are addressed by an eight-bit binary number when functioning on an H1 segment. This binary number field naturally supports a maximum addressing range of 0 to 255 (decimal), or 00 to FF hexadecimal. This address range is divided into the following sub-ranges by the Fieldbus Foundation:


Address range


Address range


0 through 15 00 through 0F Reserved
16 through 247 10 through F7 Permanent devices
248 through 251 F8 through FB New or decommissioned devices
252 through 255 FC through FF Temporary (“visitor”) devices


Devices are usually assigned addresses to function on the segment by the host system (typically a DCS with FF capability), although it is possible to order FF instruments pre-configured at the factory with addresses specified by the customer upon order. Host systems are generally configured to automatically determine device addresses rather than require the technician or engineer to manually assign each address. This make the commissioning process more convenient.

The maximum number of “permanent” devices (installed field instruments) allowed on an H1 segment for operational reasons is 32, and as you can see the addressing scheme offers far more valid addresses than that. One of the many tasks given to a segment’s Link Active Scheduler (LAS) device is to probe for new devices connected to the segment. This is done on a one-at-a-time basis, with the LAS sequentially polling for uncommissioned addresses within the valid address range. Obviously, this can be a waste of time with only 32 addresses capable of active service at any given time and over 200 valid address numbers. A practical solution to this problem is to specify an “unused” address range for the LAS to skip, so it does not waste time probing for devices (nodes) within a certain range. This address range is specified as a set of two numbers: one for the First Unused Node (abbreviated FUN), and another specifying the Number of Unused Nodes (abbreviated NUN). For example, if one wished to have the LAS on a particular H1 segment skip device addresses 40 through 211, one would configure the FUN to equal 40 and the NUN to equal 172, since the address range 40 through 211 is one hundred seventy two addresses (inclusive of both 40 and 211).

Even with a maximum operational limit of 32 devices to an H1 segment, it is rare to find segments operating with more than 16 devices. One reason for this is speed: with additional devices requiring time to broadcast and process data, the total macrocycle time (the time period between guaranteed delivery of the same process data from any one device – the determinism time) must necessarily increase. According to the Fieldbus Foundation’s engineering recommendations guide, there must be no more than twelve devices on a segment (including no more than two final control elements) in order to achieve a 1-second or less macrocycle time. For half-second update times, the recommended maximum is six devices (with no more than two final control elements). For quarter-second update times, the limit drops to a total of three devices, with no more than one final control element. Macrocycle time is essentially dead time, which is worse than lag time for any form of feedback control. When controlling certain fast processes (such as liquid pressure or flow rate), dead times on the order of one second are a recipe for instability.

Another limitation to the number of operational addresses on an H1 segment is current draw. FF devices draw 10 mA of current minimum. A FF segment with sixteen parallel-connected devices would see a total current of 160 mA minimum, with a more realistic value being in excess of 300mA.

In addition to network addresses, each FF device bears an absolutely unique identifier (a 32-byte binary number) to distinguish it from any other FF device in existence. This identifier serves much the same purpose as a MAC address on an Ethernet device. However, the identifier field for FF devices allows a far greater instrument count than Ethernet: 32 bytes for FF instruments versus 48 bits for Ethernet devices. While the Ethernet MAC address field only allows for a paltry 2.815 × 1014 unique devices, the FF identifier allows 1.158 × 1077 devices! The distinction between a FF device’s network address and the device’s identifier is virtually identical to the distinction between an Ethernet device’s IP address assigned by the end-user and its MAC address number assigned by the manufacturer.

This identifier value is usually expressed as 32 ASCII-encoded characters for brevity (one alphanumeric character per byte), and is subdivided into byte groups as follows:


First 6 bytes Middle 4 bytes

Last 22 bytes

Manufacturer code Device type code Serial number

For example, the identifiers for all Fisher brand devices begin with the first six characters 005100. The identifiers for all Smar devices begin with the characters 000302. The identifiers for all Rosemount3 brand devices begin with 001151. A typical identifier (this particular one for a Fisher model DVC5000f valve positioner) appears here:

 005100 0100 FISHERDVC0440761498160

Normally, these identifiers appear as 32-character strings, without spaces at all. I have inserted spaces within this string to make the character groupings easier to see.


Communication management

In a FF network segment, the Link Active Scheduler (LAS) device coordinates all communications between segment devices. Among the many responsibilities the LAS is tasked with are the following:

  • Commands non-LAS devices to broadcast data to the segment with “Compel Data” (CD) messages, issued at regular time intervals to specific devices (one at a time)
  • Grants permission for non-LAS devices to communicate with “Pass Token” (PT) messages, issued during unscheduled time slots to specific devices (one at a time, in ascending order of address number)
  • Keeps all segment devices synchronized with a regular “Time Distribution” (TD) message
  • Probes for new devices on the segment with a “Probe Node” (PN) message
  • Maintains and publishes a list of all active devices on the network (the Live List)


Scheduled versus unscheduled communication

As previously mentioned, Fieldbus H1 network communication may be divided into two broad categories: scheduled (cyclic) and unscheduled (acyclic). Scheduled communication events are reserved for exchanging critical control data such as process variable measurements, cascaded setpoints, and valve position commands. These scheduled communications happen on a regular, timed schedule so that loop determinism is guaranteed. Unscheduled communications, by contrast, are the way in which all other data is communicated along an H1 segment. Manual setpoint changes, configuration updates, alarms, and other data transfers of lesser importance are exchanged between devices in the times between scheduled communication events.

Both forms of communication are orchestrated by the Link Active Scheduler (LAS) device, of which there is but one active at any given time4 on an H1 segment. The LAS issues “token” messages to non-LAS devices commanding (or merely authorizing) them to broadcast to the segment one at a time. Each token message issued by the LAS grants transmission rights to an FF device either for a limited purpose (i.e. the precise message to be transmitted) or for a limited time (i.e. giving that device the freedom to transmit whatever data it desires for a short duration), after which transmission rights return to the LAS. CD tokens are message-specific: each one issued by the LAS commands a single device to immediately respond with a broadcast of some specific data. This is how scheduled (cyclic) communication is managed. PT tokens are time-specific: each one issued by the LAS grants a single device free time to transmit data of lesser importance. This is how unscheduled (acyclic) communication between devices is managed.

The LAS also issues at third type of token message: the “Probe Node” (PN) token intended to elicit a response from any new devices connected to the network segment.

In addition to transmitting tokens – which by definition are messages granting another device permission to transmit to the network – the LAS also broadcasts other messages necessary for the function of an H1 segment. For example, the “Time Distribution” (TD) message regularly broadcast by the LAS keeps all devices’ internal clocks synchronized, which is important for the coordinated transfer of data.

One of the “internal” tasks of the LAS (not requiring network broadcasts) is the maintenance of the Live List, which is a list of all known devices functioning on the network segment. New devices responding to “Probe Node” messages will be added to the Live List when detected. Devices failing to return or use PT tokens issued to them are removed from the Live List after a number of attempts. When “backup” LAS devices exist on the segment, the LAS also publishes updated copies of the Live List to them, so they will have the most up-to-date version should the need arise to take over for the original LAS (in the event of an LAS device failure).

In “busy” H1 segments where multiple devices are exchanging data with each other, a heavy traffic load of scheduled communications (CD tokens and their responses) makes it difficult for substantial unscheduled (acyclic) data exchanges to occur. For example, if a device happens to be maintaining a lengthy list of client/server requests in its queue, which it may address only during its allotted acyclic time slots (i.e. when it has been given the PT token from the LAS), it is quite possible the PT token will expire before all the device’s transactions have been completed. This means the device will have to wait for the next acyclic period before it can complete all the unscheduled communication tasks in its queue. The Fieldbus Foundation recommends new H1 segments be configured for no more than 30% scheduled communications during each macrocycle (70% unscheduled time). This should leave plenty of “free time” for all necessary acyclic communications to take place without having to routinely wait multiple macrocycles.

Virtual Communication Relationships

A term you will frequently encounter in FF literature is VCR, or “Virtual Communication Relationships.” There are three different types of VCRs in FF, describing three different ways in which data is communicated between FF devices:

  •  Publisher / Subscriber (scheduled), otherwise known as Buffered Network-Scheduled Unidirectional (BNU)
  • Client / Server (unscheduled), otherwise known as Queued User-Triggered Bidirectional (QUB)
  • Source / Sink (unscheduled), otherwise known as Queued User-Triggered Unidirectional (QUU)

Publisher / Subscriber: this VCR describes the action of a Compel Data token. The Link Active Scheduler (LAS) calls upon a specific device on the network to transmit specific data for a time-critical control purpose. When the addressed device responds with its data, multiple devices on the network “subscribing” to this published data receive it simultaneously. The publisher/subscriber VCR model is highly deterministic.

Client / Server: this VCR describes one class of unscheduled communications, permitted when a device receives a Pass Token (PT) message from the LAS. Each device maintains a queue (list) of data requests issued by other devices (clients), and responds to them in order as soon as it receives the Pass Token. By responding to client requests, the device acts as a server. Likewise, each device can use this time to act as a client, posting their own requests to other devices, which will act as servers when they receive the PT token from the LAS. This is how non-critical messages such as maintenance and device configuration data, operator setpoint changes, diagnostic messages, alarm acknowledgments and PID tuning values, etc. are exchanged between devices on an H1 segment Client/server communications are checked for data corruption by their receivers, to ensure reliable data flow.

Source / Sink (also called Report Distribution): this VCR describes another class of unscheduled communications, permitted when a device receives a Pass Token (PT) message from the LAS. This is where a device broadcasts data out to a “group address” representing many devices. Source/sink communications are not checked for data corruption, as are client/server communications.

An analogy for making sense of VCRs is to imagine lines drawn between FF devices on a segment to connect their various messages to other devices. Each line represents an individual transmission which must take place some time during the macrocycle. Each line is a VCR, some handled differently than others, some more critical than others, but all are nothing more than communication events in time. Later in this chapter, when you see function blocks connected together to form working control systems, think of the lines connecting blocks in different devices as VCRs5.


Device capability

Not all FF devices are equally capable in terms of Data Link (layer 2) functions. The FF standard divides data link device functionality into three distinct groups, shown here in order of increasing capability:

  • Basic devices
  • Link Master devices
  • Bridge devices

A Basic device is one capable of receiving and responding to tokens issued by the Link Active Scheduler (LAS) device. As discussed previously, these tokens may take the form of Compel Data (CD) messages which command immediate response from the Basic device, or Pass Token (PT) messages which grant the Basic device time-limited access to the segment for use in broadcasting data of lesser importance.

A Link Master device is one with the ability to be configured as the LAS for a segment. Not all FF devices have this ability, due to limited processing capability, memory, or both6.

A Bridge device links multiple H1 segments together to form a larger network. Field instruments are never Bridge devices – a Bridge is a special-purpose device built for the express purpose of joining two or more H1 network segments.


1While many industrial control systems have been built using networks that are not strictly deterministic (e.g. Ethernet), generally good control behavior will result if the network latency time is arbitrarily short. Lack of “hard” determinism is more of a problem in safety shutdown systems where the system must respond within a certain amount of time in order to be effective in its safety function.

2By “sequencing,” I mean the execution of all antecedent control functions prior to “downstream” functions requiring the processed data. If in a chain of function blocks we have some blocks lagging in their execution, other blocks relying on the output signals of those lagging blocks will be functioning on “old” data. This effectively adds dead time to the control system as a whole. The more antecedent blocks in the chain that lag in time behind the needs of their consequent blocks, the more dead time will be present in the entire system. To illustrate, if block A feeds data into block B which feeds data into block C, but the blocks are executed in reverse order (C, then B, then A) on the same period, a lag time of three whole execution periods will be manifest by the A-B-C algorithm.

3The engineers there are not without a sense of humor, choosing for their manufacturer code the same model number as the venerable 1151 differential pressure transmitter, perhaps the most popular Rosemount industrial instrument in the company’s history!

4In addition to the main LAS, there may be “backup” LAS devices waiting ready to take over in the event the main LAS fails for any reason. These are Link Master devices configured to act as redundant Link Active Schedulers should the need arise. However, at any given time there will be only one LAS.

5In the specific case of function block connections, each of those lines is a Publisher/Subscriber VCR, because each of those lines represents critical control data that must be passed from device to device in order for the function-block “program” to perform its control task.

6Some FF devices capable of performing advanced function block algorithms for certain process control schemes may have the raw computational power to be an LAS, but the manufacturer has decided not to make them Link Master capable simply to allow their computational power to be devoted to the function block processing rather than split between function block tasks and LAS tasks.


Click here to go the next page, FOUNDATION Fieldbus Function Blocks

Click here to go back to the previous page, H1 FOUNDATION Fieldbus Physical layer


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