Overview
Most device registers are written and/or read by address. Buffer registers are special registers that are used when multiple values must be written or read, but the number of values are able to change. Buffer Registers produce multiple values when being read and consume all values being written, and they allow users to write a sequence of values to a single Modbus address. Buffer registers typically have a companion "_SIZE" register that defines how many sequential values should be sent to or read from a buffer register.
Buffer registers could also be called array registers; the array size is defined using a "_SIZE" register, then the array data is accessed or modified using a single specific (Modbus) address.
For example, consider the difference between AIN0 and FILE_IO_PATH_READ:
Normal register:
- AIN0 is at address 0 and is followed by AIN1 at address 2
- AIN0 is a normal register
- Reading an array of 4 registers starting at address 0 would read 2 registers from AIN0 and 2 registers AIN1 (AIN values are FLOAT32, which each consist of 2 registers)
Buffer register:
- FILE_IO_PATH_READ is at address 60652 and is followed by FILE_IO_WRITE at address 60654
- FILE_IO_PATH_READ is a Buffer Register
- Reading an array of 4 registers starting at address 60652 would read 4 registers from FILE_IO_PATH_READ. FILE_IO_WRITE would not be read.
- Note that users would first designate that 8 bytes are about to be read by writing a value of 8 to FILE_IO_PATH_READ_LEN_BYTES.
In practice, the important differences are:
- You don't need to know what registers follow a Buffer Register, you can simply write / read without worrying about colliding with other registers
- You can only write / read values sequentially. E.g. you cannot modify previously written values.
- Define how much data to send/receive to/from the buffer register using the associated _NUM_BYTES, or _SIZE, or _LEN register.
- Often it is necessary to complete the transaction with an action register, such as _GO, or _OPEN, or _ENABLE.
Buffer Registers, and their size definitions:
Serial Comm Systems
- ASYNCH_DATA_RX
- ASYNCH_DATA_TX
- ASYNCH_NUM_BYTES_RX
- ASYNCH_NUM_BYTES_TX
- I2C_DATA_RX
- I2C_DATA_TX
- I2C_NUM_BYTES_RX
- I2C_NUM_BYTES_TX
- ONEWIRE_DATA_RX
- ONEWIRE_DATA_TX
- ONEWIRE_NUM_BYTES_RX
- ONEWIRE_NUM_BYTES_TX
- SPI_DATA_RX
- SPI_DATA_TX
- SPI_NUM_BYTES
File IO System
- FILE_IO_PATH_READ
- FILE_IO_PATH_WRITE
- FILE_IO_PATH_READ_LEN_BYTES
- FILE_IO_PATH_WRITE_LEN_BYTES
- FILE_IO_READ
- FILE_IO_WRITE
- FILE_IO_SIZE_BYTES
Lua Scripts/Debug Info
- LUA_SOURCE_WRITE
- LUA_SOURCE_SIZE
- LUA_DEBUG_DATA
- LUA_DEBUG_NUM_BYTES
Stream Out System
- STREAM_OUT#(0:3)_BUFFER_F32
- STREAM_OUT#(0:3)_BUFFER_U16
- STREAM_OUT#(0:3)_BUFFER_U32
- STREAM_OUT#(0:3)_BUFFER_ALLOCATE_NUM_BYTES
User RAM FIFOs
- USER_RAM_FIFO#(0:3)_DATA_F32
- USER_RAM_FIFO#(0:3)_DATA_I32
- USER_RAM_FIFO#(0:3)_DATA_U16
- USER_RAM_FIFO#(0:3)_DATA_U32
- USER_RAM_FIFO#(0:3)_ALLOCATE_NUM_BYTES
WIFI
- WIFI_SCAN_DATA
- WIFI_SCAN_NUM_BYTES
Internal Flash
- INTERNAL_FLASH_READ
- INTERNAL_FLASH_WRITE
Buffer registers can be identified using the Modbus Map tool. Expand the details button to see whether a register is a buffer register, and other details.
LJM
When using the LJM library, the ByteArray or Array functions should be used to read or write buffer buffer registers:
- Non-BYTE registers: LJM_eReadAddressArray, LJM_eReadNameArray, LJM_eWriteAddressArray, or LJM_eWriteNameArray
- BYTE-type registers: LJM_eReadAddressByteArray, LJM_eReadNameByteArray, LJM_eWriteAddressByteArray, or LJM_eWriteNameByteArray
Reading buffer registers with the wrong LJM function can result in error LJME_FUNCTION_DOES_NOT_SUPPORT_THIS_TYPE.
