88 • Appendix A - MODBUS RTU Protocol
03 Read Holding Registers. Read setting parameters.
Read actual data (for Modbus Plus users).
04 Read Input Registers. Read actual data.
05 Force Single Coil. Force one discrete command.
06 Preset Single Register. Write one parameter setting.
08 Diagnostics. Loopback diagnostics.
15 Force Multiple Coils. Force discrete commands.
16 Force Multiple Registers Write parameter setting
Control commands
DATA
Data field includes information transferred to and from the ASTAT-XT. The specific data format is changed with
Function. When Word data parameters are transmitted, High Byte is transmitted first, followed by the Low Byte.
CRC
The CRC (Cyclic Redundancy Check) two bytes (16 bit) are used to check the bytes of the entire frame.
It is generated in the master device and transmitted as the last two bytes of the frame (Low byte is appended first,
followed by the High byte). The slave device regenerates the CRC bytes and compares it to the CRC bytes received.
If the CRC bytes are not identical, the frame is flushed and no response is transmitted to the master.
ASTAT-XT MEMORY ORGANIZATION
The ASTAT-XT memory is organized according to the common Modbus addresses as follows:
ASTAT-XT Use Memory Type Max Query/Response Parameters
Actual Data 3X References Registers, # 1...128, addressed 0..127.
4X References Registers, #257...384 addressed. 256...383
Setting Parameters 4X References Registers, # 1...109, addressed 0...108
Hardwired Control Inputs 1X References 16 Inputs, # 1..16, addressed 0..15.
Discrete Serial Commands 0X References 16 Coils, # 1..16, addressed 0..15.
Control Commands 4X References 1 Register, # 753, addressed 752.
Notes:
1. Actual Data parameters can be read both at 3X references starting at parameter #1, or (same parameters)
at 4X references starting at parameter #257 (100 hex higher). The additional mapping in 4X references is designed
for the convenience of Modbus Plus users.
2. ASTAT-XT can be controlled using the standard discrete commands (Coils, 0x references) or by writing to
parameter setting #753 with function 16. The additional control option using 4X references is designed for the
convenience of Modbus Plus users. To control, write to register #753 (address 752) only! (one register write, with
function 16).
3. Function 3 should be used to read 4X references. Function 4, to read 3x references.
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