Time-Triggered Communication - Networked Embedded Systems - page 337

14.5.2.2 Periodic Exchange of State Messages
The TTP/A protocol distinguishes two types of communication rounds, namely “multipartner
rounds” and “master/slave rounds.” Multipartner rounds serve the purpose of periodically trans-
mitting messages with state information. A multipartner round consists of a configurable number
of slots with a specific sender node for each slot. The configuration of a round is defined at design
time using a static data structure. This data structure defines which node transmits in a slot, the
operation of each individual slot, and the receiving nodes. As the current global time in a TTP/A
system unambiguously identifies a message, no explicit message name (e.g., message identifier) is
required in messages.
Master-slaveroundsallowtoreaddatafromanIFSilerecord,towritedatatoanIFSilerecord,
or to execute a selected IFS file record within the cluster. A master-slave round consists of two
phases, an address phase, and a data phase. During the address phase the master specifies (in a
message to the slave node) which type of file operation is intended (read, write, or execute) and
the address of the selected file record. Master-slave rounds can be employed for inherently event-
triggered activities like monitoring, maintenance, or configuration. A maintenance engineer can
open an event-triggered communication channel to a node by using corresponding tools as described
in Ref. [Pit].
14.5.2.3 Fault Isolation Mechanisms
TTP/A focuses on low-cost fieldbus networks, thus providing limited error detection and fault
isolation. TTP/A includes a parity bit in every byte and offers the flexibility of byte-protected,
nibble protected, or unprotected data fields. TTP/A assumes that the application will pro-
vide an application-specific end-to-end protection for data fields that are not protected by the
protocol.
TTP/A does not employ bus guardians and assumes that faulty nodes will exhibit a single simple
external failure mode: fail-silence. The resulting assumption coverage is adequate for non-safety-
critical, low-cost fieldbus applications.
14.5.2.4 Diagnostic Services
TTP/A specifies a Diagnostic and Maintenance (DM) interface, which opens a communication chan-
nel to the internals of a node for the purpose of DM. This interface is used to set parameters and
to retrieve information about the internals of a node, e.g., for the purpose of fault diagnosis. The
maintenance engineer who accesses the sensor internals via the DM interface is required to have
detailed knowledge about the internal structure and behavior of the node. he DM interface is avail-
able during system operation without disturbing the real-time service. To achieve the objective of
supporting online maintenance without probe-effects, the sporadic diagnostic traffic is implemented
using master-slave rounds. hereby, it coexists with the real-time traffic (which is implemented using
multipartner rounds) without disturbing the latter.
14.5.2.5 Commercial or Prototypical Components
Several prototype implementations are available for embedded computing nodes based on the TTP/A
protocol. For example, Ref. [Elm] describes an implementation of a TTP/A node using the -bit
microcontroller Atmel AVR ATS (cf. Figure .). .).This microcontroller has  KByte of Flash
memory and  Bytes of RAM. The physical network interface is implemented using an ISO 
k-line bus, namely, a single wire bus with a bandwidth up to  kbps. In addition, implementations
using other Atmel microcontrollers (e.g., Atmel AVR ATS or ATX) and Microchip PIC
microcontrollers (e.g., PIC F) have been realized [Tro].