Appendix E Falcon Lead Lag
E-67
Application Layer
The Falcon Modbus interface supports the following function codes:
• 03 (0x03) Read Holding Registers
• 06 (0x06) Write Single Register
• 16 (0x10) Write Multiple Registers
• 17 (0x11) Report Slave ID
All the configuration and status data are accessed as 16-bit holding registers in this interface. Since
all Falcon digital signals accessed in this interface are read only, these digital signals are mapped to
bits within holding registers instead of coils or discrete inputs, to simplify the interface. Variable
length data are also represented by holding registers and therefore must be accessed individually and
not as part of a group. The length of the variable length data is returned in the response. All 32-bit
data items are accessed as two consecutive, 16-bit holding registers, i.e., each item uses 2 register
address spaces.
Except for variable length data items the registers can be accessed as a single register or up to 20
registers for writes and 125 registers for reads. Data is mapped into logical groups with room for
future expansion, so some gaps exist in the register map.
Data organization is intended to allow for efficient register access. Status data is organized into regis-
ter blocks by application function and a function status change indicator is used to denote when any
data has changed within the register block since the last time the registers were read (See Fig. 1).
The Falcon sets the status change indicator bit when at least one of the registers in the functional
block has changed value since it was last read. The Modbus master can read the status change reg-
ister and determine which functional register blocks have changed value since its last access and
only read those register blocks. The Modbus master can ignore the status change register and poll
status data as it deems fit.
Table 5: RS-485 message format
Coding system 8-bit binary
Number of data bits per character 10 =
1 start bit
8 data bits
No parity bit
1 stop bit
Bit transfer rate 38400 bps
Duplex Half duplex
Error checking 2 byte CRC-16 polynomial
Bit transfer order LSB first
End of message Idle line for 3.5 or more characters
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