Ethernet Communications Modules, 3rd Edition, Rev. D
4–11
Chapter 4: RLL Programming for Communications
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A
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D
Special Relays for Communications
The
Direct
LOGIC PLCs provide internal contacts (bits) for monitoring the status of
communications. The internal contacts are called Special Relays (there are other Special Relays
used for other purposes). There are two Special Relays for each slot in the base that will accept
the ECOM module. The two relays perform the following functions:
• Communication Busy – This bit is on when the communication module
is busy transmitting or receiving. You must use this bit, or relay contact,
to prevent overwriting your Read or Write (RX/WX) instructions.
• Communication Error – This bit is on when an error occurred in the
last RX or WX communication. This error automatically clears (the bit
resets to zero) when another RX or WX instruction executes.
For example, Special Relays SP124 and SP125
correspond to an ECOM module in slot 3 of the PLC
base.
The Special Relay SP125 is used in the example to
energize the output Y50, indicating a communication
error has occurred. This Special Relay must appear
earlier in the program than your RX or WX
instruction because it is turned off (reset to zero) when
a subsequent Read or Write instruction is executed.
The Special Relay SP124 indicates the ECOM is busy.
When SP124 is on, the normally closed contact opens
to prevent executing another RX or WX instruction
until the last one is completed. The appropriate busy
bit must be used as a NC contact on every RX/WX
instruction rung in the program.
Y50
SP125
SP124
LD
LD
LDA
RX
SET
K204
DL06 Special Purpose Communication Relays
CPU Base
Slot 1
Slot 2
Slot 3
Slot 4
Communication Busy
SP120
SP122
SP124
SP126
Communication Error
SP121
SP123
SP125
SP127
DL05 Special Purpose Communication
Relays
CPU Base
Option Slot
Communication Busy
SP120
Communication Error
SP121
