61
Status Reception Problems
The RAM in the PCU that contains status data is accessed both by the PCU
to write the status and by the PC to read status (READ(88)). Both of these
operation cannot take place at the same time.
ÉÉÉÉÉÉ
ÉÉÉÉÉÉ
EEPROM
PC
NC222-E
Every 10 ms
ÉÉÉÉÉÉ
ÉÉÉÉÉÉ
RAM
ÉÉÉÉÉÉ
READ(88) sent
Status
Status
READ(88) response
If a READ(88) is attempted while the PCU is writing status, READ(88) will be
ignored. The status write operation is performed approximately every 10 ms
and requires approximately 1 ms to complete, i.e., there is up to about a 1 in
10 chance that READ(88) will not be executed as programmed. This situation
is illustrated below.
Writing status to RAM (PCU)
Approx. 10 ms
READ(88) (reading status from PC)
Scan time
Not executed
A
B
C
D
E
F
G
A
B
E
F
Status stored in PC
If it is necessary to ensure that status is read during each program scan,
block programming can be used. One example of how to do this is shown
below. This example reads out 17 status words to IR 100 to IR 116 for a PCU
allocated IR 000 and IR 001. The loop in the block program will be taken until
the Equals Flag (=) turns ON (i.e., until IR 1200 turns ON).
BPRG(96) 00
25313
(Always ON)
LOOP <09>
READ(88)
#0017
001
LEND <10>
25506
BEND <01>
(Equals Flag)
PC Programs
Section 4-2
