78
6-1
Timing Considerations
READ(88/190) is the I/O READ instruction and WRIT(87/191) is I/O WRITE in-
struction. These are PC commands and are executed from within the PC ladder
diagram program. READ(88/190) and WRIT(87/191) give the PC the ability to
transfer large blocks of data during one cycle time: up to 255 words at a time. The
MOV(21/030) instruction can only transfer one word of data per cycle.
Because variable sized blocks of data can be transferred with one
READ(88/190)/WRIT(87/191) instruction, the amount of time needed to com-
plete execution of the READ(88/190)/WRIT(87/191) instruction will vary de-
pending on how many words of data are being transferred. Therefore, the PC
must have a method of informing the ASCII Unit when the data transfer opera-
tion is completed. The PC uses the Equals Flag for this purpose. When the PC is
in the midst of executing a READ(88/190)/WRIT(87/191) instruction, this flag is
turned OFF. When the READ(88/190)/WRIT(87/191) instruction finishes ex-
ecuting, this flag is turned ON.
The diagram on the following page illustrates the timing relationships between
READ(88/190) and the PC WRITE statement and WRIT(87/191) and the PC
READ statement.
Whenever the ASCII Unit is writing data with the PC WRITE statement, the PC is
reading data with READ(88/190) and whenever the PC is writing data with
WRIT(87/191), the ASCII Unit is reading data with the PC READ statement. This
illustrates two important points:
•
Whenever the ASCII Unit and the PC communicate, one of them is reading and
the other one is writing.
•
The device which is writing data always initiates data transfer.
The following rules are illustrated in the diagram on the next page:
B.
If a READ(88/190) is executed before its corresponding PC WRITE
statement, it is treated as a NOP.
3.
If a PC WRITE statement is executed before processing of a pre-
vious PC WRITE statement is completed, it must wait for execution
of the next READ(88/190) before data transfer can begin.
6.
If a PC READ statement is executed before processing of a pre-
vious PC READ statement is completed, it must wait for the next
WRIT(87/191).
H.
If a WRIT(87/191) instruction is executed before processing of the
previous WRIT(87/191) instruction is completed, it is treated as a
NOP.
Timing Considerations
Section 6-1
Содержание C500-ASC04
Страница 1: ...C500 ASC04 ASCII Unit Operation Manual Revised February 2001 ...
Страница 5: ...iv ...
Страница 7: ...vi ...