Specifications

Section 2—Modbus Plus Overview Bulletin No. VVDED397044US R12/00

MSTR Block December 2000

As a node on a MODBUS PLUS network, the ATV58 drive controller

cannot initiate messages, but all of its command (read and write),

adjustment (read and write), and display (read only) registers can be read

by other networked devices through messaging at any time, even when

the drive controller is running.

Other networked devices can write to:

• Command registers, as long as they are not peer cop registers and the

command semaphore is not taken

• Adjustment registers, as long as they are not peer copped

Other nodes cannot write to peer copped registers via message handling

except in the event of a peer cop time-out. If a write message is received

for peer copped registers, the message is refused and a negative

acknowledgment is returned.

MSTR BLOCK

Overview of MSTR Block

PLCs that support MODBUS PLUS communications have a special

MSTR (master) instruction with which nodes of the network can initiate

message transactions. The MSTR function allows you to initiate one of

nine possible network communications operations. Each operation is

designated by a code (see Table 10):

This section discusses read and write MSTR instruction blocks. For

additional information on Modbus instructions, refer to the

Ladder Logic

Block Library User Guide,

840 USE 10 100.

Table 10: MSTR Operation Codes

MSTR Operation Code MSTR Operation Code

Write Data 1 Get Remote Statistics 7

Read Data 2 Clear Remote Statistics 8

Get Local Statistics 3 Peer Cop Status 9

Write Global Database 5

Read Global Database 6