Echelon Neuron C - Manual (Page 204)

192 Memory Management
If explicit addressing is used, add an additional 11 bytes of system overhead.
For application messages,
message_size equals 1 byte for the message code plus
the number of bytes of data. For network variables, message_size equals 2 bytes
plus the number of bytes in the network variable.
Table 14 on page 196 lists the valid sizes for application buffers. For example, if
message_size is 40, then you need an application buffer of at least 45 bytes.
However, the next largest valid size for an application buffer is 50 bytes.
Application buffers are also used to receive network management messages used
by network tools for device configuration. A minimum input application buffer
size of 22 bytes (34 bytes if explicit addressing is used) is required to be able to
accommodate the largest possible network management messages.
Network Buffer Size
The size of a network buffer is less than or equal to the following:
message_size + 6 bytes of system overhead + 20 bytes of protocol overhead
Protocol overhead ranges from 5 to 20 bytes per message, and this formula uses
the maximum. A 40-byte message would thus need a network buffer of at least
66 bytes (see Table 14 on page 196).
Network buffers are also used to receive and respond to network management
messages that are used by network tools for device configuration. A minimum
input network buffer size of 42 bytes and output network buffer size of 50 bytes is
required to be able to accommodate the largest possible network management
messages.
Errors
If an input message fits into a network input buffer but does not fit into an
application input buffer, the message is discarded. An APP_BUF_TOO_SMALL
error code is logged by the Neuron firmware. If the message was sent with the
acknowledged service, no acknowledgment is sent. If the message was a request,
no response is sent.
If an output message fits into an application output buffer but does not fit into a
network output buffer, a NET_BUF_TOO_SMALL error is logged and the device
resets.
Buffer Counts
In most cases, the default number of output application buffers is sufficient.
Increasing the number of application buffers on the output side decreases the
likelihood of entering preemption mode if you are using synchronous network
variable outputs (see Preemption Mode on page 55).
The number of input network buffers needed is a function of the types of service
used and the types of connections between devices:
• If you are using authentication, you might need to increase the number of
network buffers because authentication doubles the number of messages.