BAS communication standards - Intelligent Buildings and Building Automation - page 83

the message are valid. Additionally, parity checking is also applied to each
transmitted character in its data frame.
5.4.3 Message structure
The Modbus application protocol uses a well-defined message format. Each
Modbus message has the same structure involving four basic elements,
including device address (address of receiving device), function code (Modbus
function code), data (data block with additional information), and error
checking . The sequence is always the same, which makes passing very fast
and efficient.
5.5 PROFIBUS and its features
PROFIBUS (PROcess Fieldbus) is a widely accepted open automation and
field network standard, which is supported by an industry supplying a wide
range of equipment, tools and support. PROFIBUS was introduced in 1989
as a German standard, DIN 19245, later adopted as International Standard
EN 50170. The PROFIBUS standard is now incorporated into IEC 61158,
the international fieldbus standard.
5.5.1 Protocol mode and technologies
The PROFIBUS family consists of three compatible versions offering very
high integrity and a capability appropriate to the need, including:
￿ PROFIBUS DP (Decentralized Periphery) providing low-cost, high-
speed, simple field-level communications. About 90 per cent of current
PROFIBUS applications use DP.
￿ PROFIBUS FMS (Fieldbus Message Specification) provides high-end,
applications-level communications. It is normally used at cell or controller
level, providing object-oriented transmission of structured data, loading
and control of programs, alarm services, and the like.
￿ PROFIBUS PA (Process Automation) has been developed specifically for
very cost-effective two-wire connection carrying both power and data.
It is particularly cost effective for hazardous environments such as in
industrial processing.
The three protocol technologies can operate together. DP and FMS share
the same electrical transmission system (RS-485), while PA uses a different
electrical transmission system (IEC 1158-2) but shares the same protocol as
DP and FMS. PROFIBUS DP extensions and the integration of PROFIBUS
with Ethernet technology indicate that FMS is less important than in the past.
FMS is no longer supported by PROFIBUS International. However, there are
still FMS installations in operation.