ModSnmp Manual v3.15 - 4.1.1. General settings
Name. Enter a unique name for the interface. This name will be used in Device definitions to identify the interface to be used.
Interface Type. Select Serial for a RS232/485 serial interface. Select TCP Socket for a TCP/IP network interface. Select UDP Socket for a UDP/IP interface (use of UDP for Modbus is non-standard). Note that the Serial option will only be available if you are using Windows or Linux (the → window tells you whether the serial comms library has been loaded).
Packet Type. You would normally select RTU if you are using a serial interface, and TCP if you are using a socket interface. You may have to use ASCII for some legacy serial interfaces or devices. It is possible to use RTU or ASCII packets over a socket interface - this is non-standard, but can be useful for software testing. It is also possible to use TCP packets over a serial interface, but there is no good reason to do so (and there would be no error-checking of the packets).
EOM Timeout. Enter the maximum delay (in milliseconds) expected within a message. The default value works fine in most situations, but you may need to increase the value if you are using a serial interface or operating system that introduces large delays in the middle of messages.
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Idle timeout. Period in seconds after which a connection will be closed if idle. A value of 0 will disable the timeout.
A connection is considered to be idle if no valid responses have been received from the slave device within the timeout period.
Setting an idle timeout can be used to avoid holding on to resources that are not currently in use. It can also be useful for recovering from some error conditions.
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Allow Request Pipelining. When using the TCP packet type, ModSnmp may send a request to a slave before responses to earlier requests have been received. This is known as "pipelining". The "transaction ID" in the request and response messages allows ModSnmp to match the responses to the corresponding requests.
Deselect Allow Request Pipelining if the slave cannot handle pipelined requests.
The Interface Type and Packet Type together determine the variant of the Modbus protocol that will be used. The table below lists the possible combinations:
Table 1. Modbus protocol variants
| Interface type | Packet type | Protocol variant |
|---|---|---|
| Serial | RTU | Modbus RTU |
| Serial | ASCII | Modbus ASCII |
| Serial | TCP | Not recommended since no error checking (non-standard) |
| TCP Socket | RTU | Modbus RTU encapsulated in TCP (non-standard) |
| TCP Socket | ASCII | Modbus ASCII encapsulated in TCP (non-standard) |
| TCP Socket | TCP | Modbus TCP |
| UDP Socket | RTU | Modbus RTU encapsulated in UDP (non-standard) |
| UDP Socket | ASCII | Modbus ASCII encapsulated in UDP (non-standard) |
| UDP Socket | TCP | Modbus UDP (also known as Modbus TCP encapsulated in UDP) (non-standard) |