Siemens SIMATIC S7-200 - Page 379

SIMATIC Instructions
9-191
S7-200 Programmable Controller System Manual
A5E00066097-02
Table 9-26 Special Memory Bytes SMB86 to SMB94, and SMB186 to SMB194 (continued)
Port 0 DescriptionPort 1
SMB87 SMB187
7
MSB
LSB
0
Receive message control byte
en: 0 =Receive Message function is disabled.
1 =Receive Message function is enabled.
The enable/disable receive message bit is checked each time
the RCV instruction is executed.
sc: 0 =Ignore SMB88 or SMB188.
1 =Use the value of SMB88 or SMB188 to detect start of
message.
ec: 0 =Ignore SMB89 or SMB189.
1 =Use the value of SMB89 or SMB189 to detect end of
message.
il: 0 =Ignore SMW90 or SMB190.
1 =Use the value of SMW90 to detect an idle line condition.
c/m: 0 =Timer is an inter-character timer.
1 =Timer is a message timer.
tmr: 0 =Ignore SMW92 or SMW192.
1 =Terminate receive if the time period in SMW92 or SMW192
is exceeded.
bk: 0 =ignore break conditions,
1 =Use break condition as start of message detection.
The bits of the message interrupt control byte are used to define the
criteria by which the message is identified. Both start of message and
end of message criteria are defined. To determine the start of a
message, either of two sets of logically ANDed start of message
criteria must be true and must occur in sequence (idle line followed
by start character, or break followed by start character). To determine
the end of a message, the enabled end of the message criteria is
logically ORed. The equations for start and stop criteria are given
below:
Start of Message = il * sc + bk * sc
End of Message = ec + tmr + maximum character count
reached
Programming the start of message criteria for:
1. Idle line detection: il=1, sc=0, bk=0, SMW90>0
2. Start character detection: il=0, sc=1, bk=0, SMW90
is a don’t care
3. Break Detection: il=0, sc=0, bk=1, SMW90
is a don’t care
4. Any response to a request: il=1, sc=0, bk=0, SMW90=0
(Message timer can be used to terminate receive if there is no
response.)
5. Break and a start character: il=0, sc=1, bk=1, SMW90
is a don’t care
6. Idle line and a start character: il=1, sc=1, bk=0, SMW90 >0
7. Idle line and start character (Illegal): il=1, sc=1, bk=0, SMW90=0
Note: Receive will automatically be terminated by an overrun or a
parity error (if enabled).
en sc ec il tmrc/m
bk
0
SMB88 SMB188 Start of message character