Watlow D4T Data Logger
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Chapter 6 Appendix
Communications
The D4T data logger is equipped with Modbus
®
TCP and Standard Commands for Programma-
ble Instruments as embedded protocols.
Introduction to Standard Commands for Programmable Instruments (SCPI)
This protocol was originally designed in the 1960s by Hewlett-Packard using the IEEE 488
standard (8-bit parallel bus) and was created for the primary purpose of allowing comput-
ers to talk with programmable instrumentation. SCPI commands are ASCII text strings with a
wide array of defined SCPI commands, all of which are not included in this implementation.
Although the SCPI protocol can be deployed over multiple physical layers Watlow has imple-
mented this protocol over Ethernet port 502. The available SCPI commands are shown below:
IEEE 488.2 commands
*IDN?
Identification Query
Commands for process value measurements
:UNIT:TEMPerature? Read communications temperature units
:UNIT:TEMPerature C Set communications temperature units to C
:UNIT:TEMPerature F Set communications temperature units to F
:UNIT:TEMPerature:DISPlay? Read displayed temperature units
:UNIT:TEMPerature:DISPlay C Set displayed temperature units to C
:UNIT:TEMPerature:DISPlay F Set displayed temperature units to F
Commands to provide access to soft key
:KEY#[:STATe]?
Query key state
:KEY#[:STATe] ON
Set key state ON
:KEY#[:STATe] OFF
Set key state OFF
:KEY#:NAME?
Query key name
Introduction to the Modbus Protocol
Gould Modicon, now called AEG Schneider, first created the protocol Referred to as Modbus
RTU used in process control systems. Modbus provides the advantage of being extremely reli-
able in exchanging information, a highly desirable feature for industrial data communications.
This protocol works on the principle of packet exchanges. The packet contains the address of
the data logger to receive the information, a command field that says what is to be done with
the information, and several fields of data. Each D4T parameter has a unique Modbus address
and they can be found in the table below.
All Modbus registers are 16-bits and are listed in the following table as relative addresses (ac-
tual). Some D4T parameters are contained within 32 bits (IEEE float, signed 32 bit), notice that
only one (low order) of the two registers is listed. By default, the low order word contains the
two low bytes of the 32-bit parameter. As an example, in the table below find the Universal
Input and then take a close look at the first member (
). Note that it lists
register 27586. Because this parameter is a float, it is actually represented by registers 27586
(low order bytes) and 27587 (high order bytes) as stated above. The Modbus specification does
not dictate which register should be high or low order therefore, Watlow provides the user the
ability to swap this order.
Chapter 6: Appendix
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