User`s manual

Watlow EZ-ZONE™ Communications • 12 • Chapter 2 Mobbus RTU & TCP
The process value of the EZ-ZONE™ PM is contained in two 16-bit registers. Register 360 contains the
two lower bytes (least significant word, LSW) while register 361 contains the two higher bytes (most signifi-
cant word, MSW). The 32-bit answer is an IEEE 754, 32-bit float data type.
As an example:
977D 429C is in Low Word – High Word Order. Changing to High Word – Low Word, the value is 429C 977D.
429C977D = 78.20407867 degrees when read as a 32-bit float
To read a 32-bit value, perform the following:
Assemble a packet (examples follow below) to send the controller based on these steps:
1. Determine controller address to read. Example: Address 1
2. Determine function code for read. Example: Function Code 3 hexadecimal for read holding register
3. Determine relative Modbus registers to read (360 & 361 decimal for Analog Input 1)
4. Convert register numbers to Hexadecimal. Example: 360 decimal = 168 hexadecimal
5. Enter 0 for number of registers to read high byte
6. Determine number of registers to read. Example: 2 registers to retrieve a 32-bit value
7. Enter number of registers to read low byte from previous step into packet.
8. Calculate the CRC on the packet.
9. Enter the Low Byte of CRC calculation into packet
10. Enter the High Byte of CRC calculation into packet
11. Send packet as one continuous stream
12. Wait for response from controller
Process the packet received based on these steps:
1. Process packet for accuracy by comparing CRC to calculated value
2. Parse answer from packet based on number of bytes returned
3. Convert answers to appropriate data type
Reading & Writing 32-Bit PM Parameters
Binary Hex Decimal Purpose
00000001 01 1 Controller Address
00000011 03 3 Function Read
00000001 01 1 Read Starting at Register High Byte (AIN 1 Process is Register 360 & 361)
01101000 68 104 Read Starting at Register Low Byte (AIN 1 Process is Register 360 & 361)
00000000 00 0 Read number of consecutive registers - High Byte (Always 0)
00000010 02 2 Read number of consecutive registers - Low Byte
10000100 44 68 Low byte of CRC
00101011 2B 43 High byte of CRC
Table 2.1
The CRC (also a 16-bit wide value) is sent in reverse order, low byte then high byte.
To acquire more information on the Modbus packet layout direct your browser to: http://www.modbus.org.