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Ethernet Communications Modules, 3rd Edition, Rev. D
4–19
Chapter 4: RLL Programming for Communications
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A
B
C
D
Second RX/WX Instruction
C50 is normally open and C51 is normally
closed. For this rung to be executed, the
C50 bit must be on and the C51 bit must
be off, corresponding to the second row of
the truth table. C50 was turned on in the
previous rung. C51 was turned off in the
previous rung.
After the WX instruction is executed C50
is RESET (turned off ) which opens the
C50 contact in this rung and closes it in
the next rung.
C51 is SET (turned on), which closes the
normally open C51 contact in the next
rung.
Third RX/WX Instruction
In this last rung, C50 is normally closed
and C51 is normally open. For this rung to
be executed, the C50 bit must be off and
the C51 bit must be on, corresponding to
the third row of the truth table. C51 was
turned on in the previous rung.
After the RX instruction is executed,
C50 is RESET which opens the C50
contact in this rung and allows it to close
in preparation for repeating the first
communication rung on the next CPU
scan (page 4-18).
C51 is also RESET, which allows the
C51 contact to close in preparation for
repeating the first communication rung
on the next CPU scan (page 4-18).
LD
K109
LD
K2
LDA
O40601
WX
VC0
SP122
SET
C51
C51
C50
RST
C50
LD
K114
LD
K2
LDA
O40602
RX
VC20
SP122
RST
C51
C51
C50
RST
C50
Returning to the First RX/WX Instruction
At the end of the third RX/WX instruction, we cycle back to the top row of the truth table
on page 4-18. Both C50 and C51 are off, and the next CPU scan executes the first RX/WX
instruction.