Installation manual

Publication 1747-RM001G-EN-P - November 2008
12-22 SLC Communication Instructions
If the message timeout is set to zero, the message instruction will never
timeout. Set the Time Out bit (TO = 1) to flush a message instruction
from its buffer if the destination device does not respond to the
communications request.
• Modbus - MB Data Address (1-65536)
Modbus addressing is limited to 16 bits per memory group, each with a
range of 1 to 65,536. There are four memory groups, one for each
function:
– coils (generally addressed as 0xxxx)
– contacts (1xxxx)
– input registers (3xxxx)
– holding registers (4xxxx)
Coils and contacts are addressed at the bit level. Coils are outputs and
can be read and written. Contacts are inputs and are read-only.
Input registers and holding registers are addressed at the word level.
Input registers are generally used for internally storing input values.
They are read-only. Holding registers are general purpose and can be
both read and written.
The most significant digit of the address is considered a prefix, and does
not get entered into the MB Data Address field when configuring the
message instruction.
When the message is sent, the address is decremented by 1 and
converted into a 4-character hex number to be transmitted via the
network (with a range of 0-FFFFh); the slave increments the address by
1, and selects the appropriate memory group based on the Modbus
function.
• Slave Node Address
This is the destination device's node number if the devices are on a
Modbus network.
TIP
Modbus protocol may not be consistently implemented in
all devices. The Modbus specification calls for the
addressing range to start at 1; however, some devices start
addressing at 0.
The Modbus Data Address in the Message Setup Screen
may need to be incremented by one to properly access a
Modbus slave’s memory, depending on that slave’s
implementation of memory addressing.