MODBUS Interface
Verbatim Owner's Manual F-7
The first 4 digits from the above example are the programming command for
RC association or RC alarm criteria. Thus, the sequence 4 5 0 1 refers to
programming for RC #01. The net address portion of this command sequence
is the remaining digits plus the ‘*’ used for delimiting. The 16 bit data register
has the address of 40001. The PLC’s node number is 2 and the Verbatim's NET
is 1.
The general net address syntax has the following form:
* NET * node * address *
where:
NET is NET Number - 0, 1, or 2
Net 0 is Physical Channels
Net 1 is Modbus
Net 2 is serial printer
node is PLC’s Node Number
Modbus - 1 to 256
address is Data Register Address - may be numbers. Syntax for the register
address is very specific to the PLC brand.
In the previous example, certain simplifications were made. Simplifications in
the net address may be made by using programmed defaults. The usual default
for the Verbatim's NET number is 1. The default node number may be set to
any value allowed by protocol. Therefore, in the previous example, the entry
may be simplified further to the following:
4 5 0 1 * 40001 * <ENTER>
F.5 General MODBUS Requirements
This appendix provides information specific to Modicon’s Modbus network.
There are sections on PLC data table addressing, net address format, and global
data. The paragraphs here describe general requirements and hints.
Modicon PLCs: Be sure that the DEF/MEM switch on compact 984 PLCs is
set to ‘MEM’. Otherwise, it will be impossible to change
network communications parameters from the defaults.
Modbus Networks: All nodes on a Modbus network must use RTU protocol.
There can be only one master on the network. The master
must be the Verbatim Autodialer.
Use the link-level timer (command 4908) to insert a delay
between query/response cycles. This will only be
necessary if talking to a slow device.
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