American Standard Code for Information Interchange (or in frequent context), Modbus ASCII. One of two Modbus serial transmission modes, where two eight-bit bytes of information are sent as two ASCII characters. Typically, most Modbus serial devices use the Modbus RTU protocol instead.

Discrete “On/Off” outputs in a Modbus slave that can be read and typically written by the Modbus master. One of four different Modbus data groups. The term “coil” originated from the first PLC applications, in which outputs of a PLC were set by energizing coils of output relays. See Modbus registers.

Change-of-Value.

Cyclic Redundancy Check. An error checksum mechanism used in Modbus RTU.

A numerical code (contained within an exception response) that explains why a successful response to a query cannot be delivered. See Exception codes.

A response sent by a Modbus slave when the query message (sent by the master) cannot be successfully delivered, for some reason. See Exception responses.

One of numerous functions defined within the Modbus specification, of which a device may implement. Each function has a numerical code. See Modbus function codes.

16-bit (2-byte) data registers in a Modbus slave that hold values that can typically be read and written by the Modbus master. Values may use different data types, such as integer, float, long, and others. One of four different Modbus data groups. See Modbus registers.

For hexadecimal. The base-16 numerical format used to describe Modbus message transactions, and sometimes used for Modbus data addressing.

Relating to Modbus, refers to the discrete “On/Off” status of digital inputs for a Modbus slave. Can be read (only) by the Modbus master. One of four different Modbus data groups. See Modbus registers.

16-bit (2-byte) data registers in a Modbus slave that hold read-only values. Values may use different data types, such as integer, float, long, or others. One of four different Modbus data groups. See Modbus registers.

In general Modbus terms, a message sent from the Modbus master to a slave, to retrieve or write a value in a data item. See Modbus messages.

In general Modbus terms, an addressable 2-byte (16-bit) memory location in a Modbus slave that can hold a data value. There are two main types of registers: input registers and holding registers.

In general Modbus terms, a message reply from the Modbus slave sent to the master, typically with a requested data value or other confirmation. See Modbus registers.

Remote Terminal Unit, or simply Modbus RTU. One of two Modbus serial transmission modes, where data is sent as two four-bit, hexadecimal characters. This provides higher throughput than using the (older) Modbus ASCII protocol for the same serial baud rate.

Specific to Modbus, status implies boolean (binary or On/Off) data, such as for Modbus data types coils and inputs. As used in Niagara, status can also mean general “health” of an object or output, such as “down,” “fault,” or “ok.”

Transmission Control Protocol (or in frequent context) Modbus TCP. An open Modbus protocol that facilitates Modbus message transfer using TCP/IP protocol and standard Ethernet networks.