Teledyne 2103Ci, Installation And Operation Manual, Page: 37 / 74

2103 Modem Module
Section 3 Modbus Protocol
3-5
SCADA systems can be relatively simple, such as one that mon-
itors the environmental conditions of a small office building, or
very complex, such as a system that monitors all the activity in a
nuclear power plant or a municipal water system.
3.5 Common Acronyms
ASCII – American Standard Code for Information Interchange
DCS – Distributed Control Systems
MTU – Master Terminal Unit
OPC – Object Linking and Embedding (OLE) for Process Control
PLC – Programmable Logic Controller
RTU – Remote Terminal Unit
SCADA – Supervisory Control And Data Acquisition
TCP/IP – Transmission Control Protocol/Internet Protocol
3.6 Register Specifications
All numbers in the Modbus registers are stored most significant
byte first. If the polling device has a byte ordering of least signif-
icant byte first (an Intel-based PC, for example), the bytes will
need to be reversed after they are received.
The Modbus ASCII address is used to index the data by modules.
Modbus ASCII address 1 contains information related to the site.
The first register contains a 16-bit integer count of the number of
modules that have data to report. The maximum number of
modules that can be supported is 4.
Modbus ASCII addresses 2 through the number of the module in
the stack (N) minus 1 contain data from the individual modules.
The Modbus ASCII addresses will be sorted by the model
number, and then by module name, which is entered by the user
through Flowlink. This allows the user to control the ordering of
the addresses and easily predict what data will be in specific reg-
isters.
Every measured parameter has a corresponding status and mea-
surement time that are updated with each measurement.
The maximum number of supported measurements from all
modules in the system is 28.
The Modbus registers are assigned within 30 seconds after the
2100 module is powered up. To conserve power for the users who
do not use Modbus communications, no Modbus registers will be
updated with sensor readings until a Modbus master communi-
cates with the 2100 module.