1.1 MODBUS Interface Overview

There are two serial interface options for communicating with the AquaVent using MODBUS protocol; RS-232 and RS-485. Solinst provides RS-232 and RS-485 Connector Cables, which are plugged into the side of the SPX Wellhead.

To connect the Connector Cables to the MODBUS customer equipment, you must ensure correct pin-to-pin connection, including a continuous external power supply to the SPX Wellhead. Power supplied to the Wellhead must be between 10V and 24V DC. See Section 3 for more information on physically installing the system.

The bottom of the SPX Wellhead connects to a AquaVent logger through a Vented Cable. The AquaVent logger contains a small battery, used only for memory and clock back-up. The Wellhead contains four user-replaceable 1.5V AA lithium batteries that power the AquaVent.


For more information about the AquaVent logger, Wellheads and Vented Cables, see the Vented Datalogger UserGuide.

The communication settings for the SPX Wellhead always use Serial, RTU mode, 8 data bits, even parity, and one stop bit. The MODBUS registers do not provide a means of changing any of these communication settings.

As the Wellhead acquires its identity from the AquaVent logger, in the absence of a functional AquaVent logger, or without setting a specific address or Baud rate, the Wellhead will power up with a default MODBUS device address of 1, and a default Baud rate of 19,200. These settings can be modified by Solinst Levelogger Water Level Datalogger PC Software (see Section 3.1).


The user is expected to have working knowledge of the MODBUS protocol. For more information on MODBUS, go to: www.modbus.org, where you can also find the references listed in Section 7.

The AquaVent only uses MODBUS functions that operate on registers (16 bits) and transfers data in Big Endian. The AquaVent only supports holding registers, and consequently only MODBUS functions that deal specifically with holding registers. The MODBUS master must be able to recognize floating point data types.


Technical Specifications
Wellhead Power Input: 10V - 24V DC
Modbus Connector Cable Length: 15 ft (4.5 m)
Maximum Vented Cable Length: 500 ft (150 m)
Wellhead Operating Temperature: -20ºC - 80ºC
Wellhead IP Rating: IP 64 (dust and splash resistant)

Table 1-1 MODBUS Technical Specifications