Watlow CLS200, MLS300 and CAS200 User Manual Page 29
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Chapter 2: Modbus-RTU Protocol
Communications Specification 23
• 8 data bits, least significant bit sent first
• 2 stop bits
• No parity
Error Check Field
Cyclical Redundancy Check (CRC)
Message Framing
Messages start with a silent interval of at least 3.5 character times. This
is most easily implemented as a multiple of character times at the baud
rate that is being used on the network (shown as T1-T2-T3-T4 in the
figure below). The first field then transmitted is the device address.
The allowable characters transmitted for all fields are hexadecimal 0 to
9, A to F. Networked devices monitor the network bus continuously,
including during the silent intervals. When the first field (the address
field) is received, each device decodes it to find out if it is the addressed
device.
Following the last transmitted character, a similar interval of at least 3.5
character times marks the end of the message. A new message can begin
after this interval.
Similarly, if a new message begins earlier than 3.5 character times
following a previous message, the receiving device will consider it a
continuation of the previous message. This will set an error, as the value
in the final CRC field will not be valid for the combined messages. A
typical message frame is shown below.
Handling the Address Field
The address field of a message frame contains 8 bits. Valid slave device
addresses are in the range of 0 to 247 decimal. The individual slave
devices are assigned addresses in the range of 1 to 247 decimal. A
master addresses a slave by placing the slave address in the address field
of the message. When the slave sends its response, it places its own
address in this address field of the response to let the master know which
slave is responding.
Handling the Function Field
The function code field of a message frame contains 8 bits. Valid codes
are in the range of 1 to 255 decimal. Not all these codes are applicable to
all controllers. Current codes are described in the Function Codes
section.
START ADDRESS FUNCTION DATA CRC CHECK END
T1-T2-T3-T4 8 Bits 8 Bits
n
* 8 Bits 16 Bits T1-T2-T3-T4
- CLS200, MLS300 1
- Communications 1
- Specification 1
- Warranty 2
- Contents 3
- Protocol Syntax 9
- Transaction Sequence 10
- Packet Format 12
- Codes in a Packet 12
- Error Checking 14
- Cyclic Redundancy Check (CRC) 15
- Examples 16
- Block Write 19
- Message Data 20
- Data for a Write Command 21
- Two-Byte Data Types 21
- Figuring Block Size 21
- Anafaze/AB Data Table Summary 22
- Number Description 24
- Address 24
- Type Number of Bytes 24
- Overview 27
- The Query-Response Cycle 28
- Serial Transmission 28
- Message Framing 29
- Contents of the Data Field 30
- CRC Error Checking 31
- Function Codes 32
- Diagnostic Subfunctions 33
- Writing Data 36
- Reading Data 36
- Write Examples 38
- Modbus-RTU Data Table Summary 39
- Chapter 3: 45
- All Units 17, 18 47
- Enhanced Features options 49
- Parameters (by number) 51
- Input Type (3) 52
- Output Type (4) 53
- Setpoint (5) 54
- Process Variable (6) 54
- Output Filter (7) 54
- Output Value (8) 55
- Alarm Deadband (12) 55
- Alarm_Status (13) 56
- Ambient Sensor Readings (15) 58
- Pulse Sample Time (16) 58
- High Process Variable (17) 58
- Low Process Variable (18) 58
- Precision (19) 59
- Cycle Time (20) 60
- Zero Calibration (21) 60
- Full Scale Calibration (22) 60
- Digital Inputs (25) 60
- Digital Outputs (26) 61
- Override Digital Input (28) 61
- Override Polarity (29) 61
- System Status (30) 62
- System Command Register (31) 62
- Data Changed Register (32) 63
- Input Units (33) 63
- EPROM Version Code (34) 64
- Options Register (35) 64
- High Reading (37) 65
- Low Reading (38) 65
- Heat/Cool Spread (39) 65
- Startup Alarm Delay (40) 66
- • Range: See tables below 67
- • Default: 0 67
- Current Segment (47) 68
- Segment Time Remaining (48) 68
- Current Cycle Number (49) 68
- Tolerance Alarm Time (50) 68
- Last Segment (51) 68
- Number of Cycles (52) 69
- Ready Setpoint (53) 69
- Ready Event States (54) 69
- Segment Setpoint (55) 70
- Segment Time (58) 72
- Tolerance (59) 72
- Ramp/Soak Flags (60) 72
- Output Limit (61) 73
- Output Limit Time (62) 73
- Alarm_Control (63) 74
- Alarm_Acknowledge (64) 74
- Alarm_Mask (65) 74
- Alarm_Enable (66) 74
- AIM Fail Output (68) 75
- Output Linearity Curve (69) 76
- SDAC Mode (70) 76
- SDAC Low Value (71) 76
- SDAC High Value (72) 76
- Save Setup to Job (73) 77
- Input Filter (74) 77
- Loop Alarm Delay (75) 77
- Loop Names (77) 77
- Channel Name (78) 78
- Manufacturing Test (80) 78
- Cascade Maximum Setpoint (89) 80
- Cascade Base Setpoint (87) 80
- Cascade Minimum Setpoint (88) 80
- Loop Status (96) 82
- Output Type/Disable (97) 82
- Output Reverse/Direct (98) 82
- Controller Type (99) 83
- Ramp/Soak Profile Number (100) 83
- Controller Address (101) 83
- Baud Rate (102) 83
- Ready Events (103) 84
- Appendix A: 85
- Communications Driver 85
- Close_Comm_Port() 86
- DownLoad() 86
- UpLoad() 87
- Glossary 89
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