ProSoft Technology MVI56-GEC - Manual (Page 81)

MVI56-GEC ♦ ControlLogix Platform Reference
Generic ASCII Ethernet Communication Module User Manual
ProSoft Technology, Inc. Page 81 of 108
October 8, 2010
Word Offset Description
Netmask 85 to 88 Network mask. Each word contains one of the values for the network
mask.
Gateway 89 to 92 Default gateway
93 to 96 Default network
97 to 100 Default subnet mask
101 to 246 Reserved for future use
247 Module Control Word contains a value of -3 to indicate build
configuration file option.
When configuring the module from the ladder logic, use numeric values to select
options (0 for "No" and non-zero for "Yes") rather than text values. After the
module receives this block, it will build the two configuration files and perform a
reboot operation. The module will then use the new configuration.
Handling Multiple Blocks
It is important to understand how to handle multiple blocks. The buffer size
supports 8192 bytes, but the module can handle only 2048 bytes. The module
can only send 200 bytes at each scan to the processor. For example, if a device
sends a message that contains 1000 bytes to the module, the module will break
the message down into 5 blocks of 200 bytes. The first four blocks will set the
number of characters parameter as -1, indicating that each block is part of the
same message. The last block sets the number of characters to 200, indicating
that there are no more blocks from that message.
The concept is the same for writing data from the processor to the module,
except that you can write up to 400 bytes at a time to the module. The module
buffers all data until it receives a block that contains the "number of characters"
parameter set to >=0. In this case, it sends all data to the client that is connected
to the server, or the server connects to the client.
Important: Messages are usually broken down into smaller frames by the IP layer in a specific
LAN or WAN according to the Maximum Transmit Unit (MTU) of the network.
For example, a message that contains 2000 bytes can be broken down into 2 messages by the IP
layer in the network (after it is sent to the module). The same issue is applied when a client sends
data to the server; although a client sends a single message to the module, it could be broken
down into smaller fragments before it gets to the module. In this case, the module would interpret it
as two different messages.
The application layer defines when a message is finished. This is the reason why the user should
consider using some sort of control so the ladder could identify different messages as actually part
of one single message. This could be accomplished either by using a specific character at the end
of each message or by using a fixed length for each message.