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This function will be used largely to report controller status information, and so a
bit set to 1 indicates that the corresponding feature is currently enabled/active,
and a bit reset to 0 indicates the opposite.
If an exact multiple of eight bits is not requested, the data is padded with trailing
zeros to preserve the 8-bit format. After the data has been transmitted, the
CRC16 value is sent.
B. Read Holding Registers (Read n Words) 03/04
The message sent to the controller to obtain the value of one or more registers is
an eight-byte message as follows:
The reply sent by the controller echoes the first 2 characters received and then
contains a single-byte data byte count, the value of which does not include either
itself or the CRC value to be sent. For this message, the count equals the number
of parameters read times two. Following the byte count, that number of
parameter values are transmitted, MSB first, followed by the CRC16.
C. Force Single Coil (Write 1 Bit) 05 B
The message received by the controller is 8 bytes long, consisting of the standard
preamble and the address of the bit to force, followed by a two-byte word whose
MSB contains the desired truth value of the bit expressed as 0xFF (TRUE) or 0x00
(FALSE).
Generally, this function will be used to control such features as Auto/Manual and
Tuning.
The normal reply sent by the controller will be a byte-for-byte echo of the
message received.
O077-4 Volume II 4-3
Addr. Func.
Addr. of
1st Word
No. of words
CRC16
3/4
HI HI HILO LO LO
Addr. Func.
Count
1st value 2nd value
Last value
CRC16
3/4
HIHI
HI HI
LOLO
LO LO
Addr. Func.
Addr. of Bit
State
CRC16
5
HI FF/00 HILO 0 LO