Instruction Manual

30
ModScan Overview
The ModScan application operates as a MODBUS master device in either RTU or ASCII transmission
modes. ModScan may be used to access and modify data points contained in one or more MODBUS slave
devices connected to the PC via a serial port, modem, or network. ModScan supports the standard
MODBUS message types 01-06, 15 & 16, as well as providing the ability for you to exercise special
features of a slave device by transmitting custom command strings and observing the response. ModScan is
a useful test and diagnostic tool for verifying the proper slave response to MODBUS queries as well as
being a low cost data collection tool for interfacing data into PC database and spreadsheet applications.
Document/View Architecture
ModScan utilizes the standard Windows Multiple-Document-Interface, (MDI), architecture for displaying
modbus data to the user. Each basic ModScan document represents a series, (array), of modbus data points
identified by the following parameters:
Slave Device Address
Represents the physical device attached to the modbus network
Data Type
Internal data representation, (i.e. input, coil, register)
Data Address
Point address within the device
Length
Number of points to scan/display
ModScan may also be utilized to represent different types of modbus data using customized graphical
objects as described in section VII.
Associated with each document is also a timer, which is used to periodically scan new data from the defined
slave and refresh the display. The modbus data definition is accessible from the ModScan menu or via edit
controls in the top splitter window of the document display. As new data is obtained from the slave device,
it is written to the bottom splitter window in one of several formats, depending upon your preference. The
size of each document display window is adjustable via the splitter control.
Modbus Data Definition
The upper half of each Document’s View represents the data selected for display, (and possible capture to a
historical data file). In most testing applications, the ModScan will only be connected to a single modbus
slave device, however, in a multidrop modbus network, there may be several devices accessible from a
single connection. The “Device Id” edit control allows you to specify the slave address for the source of the
data. Likewise, edit controls are available to select the point type, data address, and number of data points
to access.
Notice that the modbus protocol uses a 5-digit representation for the slave data address which infers the
point-type. For example, INPUT STATUS values are always represented in the range 10001-19999:
HOLDING REGISTERS are displayed as 40000-49999. The ModScan application uses the standard
notation for displaying data in the bottom splitter window, however the address specified in the upper
splitter address edit control assumes a 4-digit physical point address. This address, coupled with the point-
type specifier completely defines the data to be accessed in the slave device.
The upper splitter window also contains two counters which are used to tally the number of data requests
made from this document to the modbus connection and the number of valid slave responses received in
reply. A button is available within the display which resets the counters associated with this document.