Rockwell Automation 20-COMM-M Modbus/TCP Adapter - Manual (Page 90)

G-4 Glossary
20-COMM-M Modbus/TCP Adapter User Manual
Publication 20COMM-UM014C-EN-P
Hold Last
When communication is disrupted (for example, a cable is disconnected),
the adapter and PowerFlex drive can respond by holding last. Hold last
results in the drive receiving the last data received via the network
connection before the disruption. If the drive was running and using the
Reference from the adapter, it will continue to run at the same Reference.
I I/O Data
I/O data, sometimes called “ implicit messages” or “input/output,” is
time-critical data such as a Logic Command and Reference. The terms
“input” and “output” are defined from the controller’s point of view. Output
is produced by the controller and consumed by the adapter. Input is
produced by the adapter and consumed by the controller.
IP Addresses
A unique IP address identifies each node on an Modbus/TCP network. An
IP address consists of 32 bits that are divided into four segments of one byte
each. It appears as four decimal integers separated by periods
(xxx.xxx.xxx.xxx). Each “xxx” can have a decimal value from 0 to 255. For
example, an IP address could be 192.168.0.1.
An IP address has two parts: a network ID and a host ID. The class of
network determines the format of the address.
The number of devices on your Modbus/TCP network will vary depending
on the number of bytes that are used for the network address. In many cases
you are given a network with a Class C address, in which the first three
bytes contain the network address (subnet mask = 255.255.255.0). This
leaves 8 bits or 256 addresses on your network. Because two addresses are
reserved for special uses (0 is an address for the network usually used by the
router, and 255 is an address for broadcast messages to all network devices),
you have 254 addresses to use on a Class C address block.
To ensure that each device on the Internet has a unique address, contact your
network administrator or Internet Service Provider for unique fixed IP
addresses. You can then set the unique IP address for the adapter by using a
BOOTP server or by manually configuring parameters in the adapter. The
adapter reads the values of these parameters only at power-up.
0 1 7 15 23 31
Class A 0 Network ID Host ID
0 1 7 15 23 31
Class B 1 0 Network ID Host ID
0 1 2 7 15 23 31
Class C 1 1 0 Network ID Host ID