Instruction manual

MODBUS System Set Up
025-9209 29
MODBUS Version of the RTU Menu
When you access the program mode for a Model 1708/1716 that has been equipped with the
MODBUS version of the RTU firmware, a shortened version of the main menu appears.
Table 13 lists this modified menu. Basically, all the menu items dealing with radio
communication or the controller addressing have been removed.
Table 13. RTU Main Menu with MODBUS Option Installed
Menu Selections
{O}utput Definition
{D}igital Input Definition
{A}nalog Input Definition
RS232 {W}atchdog...
{F}actory defaults
E{x}it
With the exception of the last two menu items ({F}actory defaults and E{x}it), typing in the
letter for a menu item opens a submenu. The I/O menu items work just as they would for a
regular RTU. The item that is new to the MODBUS version is the RS232 {W}atchdog item.
This is the same watchdog feature offered in the Model 1700 controller menu. Since the RTU
is connected to the master PC in this configuration, it has the same need to alert the system
operator if the serial communication link to the master PC is lost.
CONFIGURING THE MASTER PC (OR PLC)
Each MODBUS software package or PLC is configured differently, so we can only give
general guidelines for configuring them here. The areas we will touch on include: serial
communications, polling intervals, timeout periods, and retries.
Serial
Communications
The serial communication parameters used by the master never
vary. They must always be set to 4800 baud, 8 data bits, no parity,
and 2 stop bits.
Polling Intervals
Report-by-Exception
In a report-by-exception system, the polling interval used by the
master can be set to a short period (on the order of 1 second). Much
of the time the Model 1700 will be responding to the polls using the
last known RTU status stored in its memory.
Polled-Only
In poll-only systems, the polling interval is generally set to a value
that allows the master enough time to have polled all the RTUs in
the system. Example: if it takes five seconds for each RTU, and the
system has 10 RTUs, the polling interval would be 50 seconds
minimum (it could be longer). Some software packages allow
different polling intervals for each RTU, so more important or