Based on the code snippet you show on your question, one gets the impression you are attempting to write a Modbus client from scratch.
Modbus is a quite simple protocol but it would take quite an effort to write an debug a new client and, since the protocol is open, there is plenty of code already written, debugged and tested. See, for instance, here.
On what command to send it's not easy to tell if you don't mention what device, in particular, you're connecting to and what exactly you want to do with it. In a frequent scenario, you have a machine or a sensor in a remote area of your factory that is logging data from its sensors and reacting from that data in a certain manner. With Modbus, you can send requests for data to your machine (reading coils/bits or registers/numeric values) to monitor what it's doing and send commands to control it (writing Modbus coils or registers) from a distant control room with an HMI or any other kind of computer.
编辑：既然您已经决定使用EasyModbus，那么您将更接近您想要的东西。但是看来您正在寻找的是从设备中读取数据，因此您无需写入寄存器。您可以尝试使用以下代码片段（源代码）：
public static void main(String[] args)
{
ModbusClient modbusClient = new ModbusClient("127.0.0.1", 1536);
try
{
modbusClient.Connect();
//Read Int value from register 0 (Barrier Command)
System.out.println(modbusClient.ReadHoldingRegisters(0, 1));
//Read Float Value from Register 1 and 2 (Barrier Status)
System.out.println(ModbusClient.ConvertRegistersToFloat(modbusClient.ReadHoldingRegisters(1, 2)));
}
catch (Exception e)
{
System.out.println(e.toString());
}
}
如果您从设备上查看Modbus地址映射（我从本手册中获取了它，并假设您的设备是该设备）：
要注意的一件事是设备上同时具有16位和32位值。由于Modbus寄存器是16位的，因此对于32位数据类型，您需要读取两个寄存器。这仅适用于编号为1的寄存器（根据以上映射的屏障状态）。对于所有其他内容，您可以读取并显示为整数值。