201
2-
11
24
Commands
INPUT telegram:
7.2.1
Data Flow Control
The data flow between a programmable logic controller (PLC) and the control
interface must be synchronized to guarantee continuous data transfer with no
losses. The input and output data is transferred in cycles. The control of the data
flow via the software is referred to as handshaking. The control bits required for
this are contained in the control byte. (see table "INPUT telegram:" on page 24
and see table "OUTPUT telegram:" on page 23)
The following handshaking is used to transfer telegrams quickly and securely
between the PLC and the control interface:
Byte
Content
Bit no.
7
6
5
4
3
2
1
0
Byte 0
Control byte
0
0
0
0
0
D
S
U
M
U
S
Byte 1
Reserved
Byte 2
Command code
Byte 3
Reserved
Byte 4
Status
Byte 5
Reply counter
Byte 6
Parameter
Byte 7
Parameter
Byte 8
Data
...
Data
Byte N
Data
Table 7.2
Slave -> master
D
S
:
Delete bit. When the bit is inverted, all expired data in the FIFO memory bank is
deleted in the slave
U
M
:
Update bit
–
master. If the master inverts this bit, it indicates the validity of a new
telegram in the OUTPUT telegram. The slave reflects this bit back inverted, thus
confirming receipt. Only then can the master (PLC) send new data.
U
S
;
Update bit
–
slave. If the slave inverts this bit, it indicates the validity of a new
telegram in the INPUT telegram. The master reflects this bit back inverted, thus
confirming receipt. Only then can the slave send new data.
