Revision 3.1 - 90 - March 2010
• Spare capacity should be defined as 20%. Therefore, if the maximum number of
field devices per segment for a project is 12, taking into account spare capacity for
future expansion, the maximum number of devices to be installed into a segment
shall be 10 at the start of the project.
• In all cases, segments shall be designed with adequate spare capacity for the future
addition of one control loop (e.g., one transmitter and one final element). Therefore,
each segment shall be designed for a minimum of two future devices.
In addition, the segment macrocycle shall be designed for this future device allowance as
described above. That is, the macrocycle shall allow one transmitter to be added (AI) and one
valve positioner (final element) to be added (AO) and still ensure that the minimum free time is
met (see section 7.7.2). This is based on a single macrocycle with default duration of one
second. Requirements for fast execution that may conflict with this directive shall be approved
by the principal.
In all spare capacity calculations, the impact on the power requirements, design of the field
junction box, segment bandwidth constraints, etc. shall be considered and sized to allow the
present and future number of devices per segment.
Commentary:
Each Host system is different in the limitations on the number of parameters communicated
to/from a segment within a given time, and hence shall be also considered in the design of
spare segment capacity.
The maximum number of VCRs allowed per segment is less of an issue with modern day Host
systems, but may be an issue in older versions.
7.7.2 Segment Execution Time
A Link Active Scheduler (LAS) is a deterministic, centralized bus scheduler that maintains a list
of transmission times for all data buffers in all devices that need to be cyclically transmitted.
The LAS, residing in the H1 interface cards, is responsible for coordinating all communication
on the fieldbus (it is in charge of the token). The cycle communication is executed at fixed
intervals called a macrocycle.
The segment macrocycles should match the execution times in the controller. The macrocycle
should have a minimum of 30% unscheduled (free asynchronous) time (see Section 4.4). The
default macrocycle time should be 1 s.
Commentary:
By exception, a faster macrocycle may be set (500 or 250 mS), but this will impact the other
segment design parameters such as default number of devices on that segment. Alternatively,
longer macrocycles are possible. The principal shall approve such deviations from the default
macrocycle time based on a dedicated segment assessment. Loop execution time must take
the location of the PID (field or Host) into consideration.