20
Communications Using Memory Links Section 1-5
In the memory link method, the only difference is that the NT31/NT31C finds
its communications target inside instead of outside. In the memory link
method, therefore, change of the target is all that is needed to be capable of
using the exact screen data that is acquired by the direct connection. (With
the NT31/NT31C, the Support Tool must be used to convert the data to
screen data for the memory link.)
When compared to the direct connection of the host link or NT link methods,
the memory link method has some restrictions regarding the use of some
functions, as described below.
• The Programming Console and Device Monitor functions cannot be used.
• The following strobes of PT status notify bits do not turn ON (page 7 to
27).
Screen switching strobe
Numerals input strobe
Character-string input strobe
• The usable area for allocation is the PT memory only.
1-5-2 Comparison between Direct Connection and Memory Link
The major differences between the direct connection and the memory link are
as follows:
• In the memory link method, the communications with the host should be
carried out by using commands. Compared to the ordinal direct connec-
tion, which can be used requiring almost no programs, the memory link
method requires a program that is necessary for interchange of com-
mands. This, however, gives advantages to the memory link method by
using a large variety of functions available in the direct connections from
major models such as a personal computer and a FA computer equipped
with RS-232C/RS-422A communications means, allowing the PT to be
suited for larger usage.
• Actual PLCs have many kinds of areas, where as PT memory is a single
area with only one kind. When creating screen data by using the memory
link methods, it is always necessary to allocate display parts in the PT
memory.
For communications between the host and the PT by the memory link method
and for handling screen display elements, refer to the NT31/NT31C/NT631/
NT631C Programmable Terminal Reference Manual.
1-5-3 Memory Link Online Transfer Function
When memory link communications are being used, the NT31/NT31C can be
switched to Transmit mode from the host even if the NT31/NT31C is operating
and screen data can be written. (The NT31/NT31C can be returned to RUN
mode after the screen data is written.)
Lamp
PT memory
PT
PLC
NT31/NT31C
Direct connection
Numeral
memory
table
RS-232C/422A
communications
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