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93
Reference Manual
00809-0100-4697, Rev GA
Appendix C: F
OUNDATION
Fieldbus Technology
November 2014
F
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Fieldbus Technology
LAS parameters
There are many bus communication parameters but only a few are used. For standard RS-232
communications, the configuration parameters are baud rate, start / stop bits, and parity. The
key parameters for H1 F
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fieldbus are as follows.

Slot Time (ST) – Used during the bus master election process. It is the maximum
amount of time permitted for device A to send a message to device B. Slot time is a
parameter which defines a worst case delay which includes internal delay in the sending
device and the receiving device. Increasing the value of ST slows down bus traffic
because a LAS device must wait longer prior to determining that the LM is down.

Minimum Inter-PDU Delay (MID) – The minimum gap between two messages on the
fieldbus segment or it is the amount of time between the last byte of one message and
the first byte of the next message. The units of the MID are octets. An octet is 256
μ
s,
hence the units for MID are approximately
1
/
4
ms. This would mean an MID of 16 would
specify approximately a minimum of 4 ms between messages on the fieldbus.
Increasing the value of MID slows down bus traffic because a larger “gap” between
messages occurs.

Maximum Response (MRD) – Defines the maximum amount of time permitted to
respond to an immediate response request, e.g. CD, PT. When a published value is
requested using the CD command, the MRD defines how long before the device
publishes the data. Increasing this parameter will slow down the bus traffic by
decreasing how fast CDs can be put onto the network. The MRD is measured in units of
ST.

Time Synchronization Class (TSC) – A variable that defines how long the device can
estimate its time before drifting out of specific limits. The LM will periodically send out
time update messages to synchronize devices on the segment. Decreasing the
parameter number increases the number of times that time distribution messages
must be published, increasing bus traffic and overhead for the LM device. See
Figure
C-3 .
Figure C-3. LAS Parameter Diagram
Backup LAS
An LM device is one that has the ability to control the communications on the bus. The LAS is the
LM capable device that is currently in control of the bus. While there can be many LM devices
acting as backups, there can only be one LAS. The LAS is typically a host system but for
stand-alone applications, a device may be providing the role of primary LAS.
C.5.2 Addressing
To setup, configure, and communicate with other devices on a segment, a device must be
assigned a permanent address. Unless requested otherwise, it is assigned a temporary address
when shipped from the factory.
FB 1
C
D
Data FB 2
MID
MID x ST
MID