Automation Direct D0-DCM, Руководство пользователя

Страница: 46 / 70

Getting Started
User Application
Guidelines
RLL
Programming
for Communications
4–8
RLL Programming for Communications
DL05/06 Data Communications Module, 1st Edition, 08/05
Special Relays for Communications
The DirectLOGIC 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 PLC that will be used by DCM module. The two relays perform the following
functions:
S
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.
S
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.
Y50 SP125
SP124
LD
LD
LDA
RX
For example, Special Relays SP124 and
SP125 correspond to an DCM module in
slot 3 of the DL06 PLC.
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
DCM 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.
SET
K304
DL05 Special Relays
Relay Option Slot
Communication busy SP120
Communication error SP121
DL06 Special Relays
Relay Slot 1 Slot 2 Slot 3 Slot 4
Communication busy SP120 SP122 SP124 SP126
Communication error SP121 SP123 SP125 SP127