ModSnmp Manual v3.09 - 4.1.4. Serial settings
The following items must be configured for a serial interface:
Parity. It's usual to use no parity, since the CRC in RTU packets is a good error check on its own, but some devices may require it. The serial specification requires even parity to be the default.
Data Bits. Must be 8 for RTU. Some ASCII devices may require 7 (the serial specification actually specifies 7 for ASCII).
Stop Bits. Usually 1, but some devices may require 2. The serial specification actually specifies 2 stop bits if parity is not being used, but this requirement is normally ignored.
Selecting Flow Control will enable hardware flow control ("handshaking") using RTS and CTS. Since Modbus messages are short, flow control is not normally necessary.
Selecting RS485 will use RTS to control a RS232 to RS485 converter: ModSnmp will raise RTS when it is transmitting data, and lower RTS to receive data. Note that the lowering of RTS may be delayed by the operating system, and this may cause loss of data. For this reason, we recommend the use of RS232-RS485 converters that do not require RTS control.