Watlow Series 988 Family - Manual (Page 25)

Sending Commands, Chapter 4 Data Communications with the Watlow Series 988 Family 4.7
Read Multiple Registers Command (0x03 or 0x04)
This command returns from 1 to 32 registers.
Packet sent to controller:| nn | 03 | nn nn | 00 nn | nn nn |
∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆
controller address (one byte)
read command (0x03 or 0x04)
starting register high byte
starting register low byte
number of registers high byte (0x00)
number of registers low byte
CRC low byte
CRC high byte
Packet returned by controller: | nn | 03 | nn | nn nn … nn nn | nn nn |
∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆
controller address (one byte)
read command (0x03 or 0x04)
number of bytes (one byte)
first register data low byte
first register data high byte
…
…
register n data high byte
register n data low byte
CRC low byte
CRC high byte
Example (988 only): Read register 0 (model number) of the controller at address 1.
Sent:
01 03 00 00 00 01 84 0A
Received: 01 03 02 03 DC B9 2D
Message: 988 (0x03DC).
Example (988 only): Read register 1 and 2 (Process 1 and 2 values) of controller at
address 5.
Sent:
05 03 00 01 00 02 94 4F
Received: 05 03 04 00 64 00 C8 FF BA
Message: 100 (0x0064) and 200 (0x00C8).
Write to a Single Register Command (0x06)
This command writes a parameter to a single register. The controller will echo back
the command. An attempt to write to a read-only parameter returns an illegal data
address error (0x02). (See “Exception Responses,” pg. 4.9.)
Packet sent to controller:| nn | 06 | nn nn | nn nn | nn nn |
∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆
controller address (one byte)
write to a register command (0x06)
register high byte
register low byte
data high byte
data low byte
CRC low byte
CRC high byte
NOTE:
Because the read
command can
only read 32
registers, the high
byte for the
number of regis-
ters will always be
0.
Modbus RTU