Watlow Series D8 - Manual (Page 75)

Series D8 User’s Guide Chapter 3: Communicating by DeviceNet
Doc. 0600-3120-2000 Watlow Anafaze 57
Table 3.2 Outbound Transaction Header
Up to 32 words are allocated for an explicit message in the
scanner used in the example. The header used 3, leaving 29 for
the message body. In this example only 4 words are used in
the message body. The first 3 words of the body contain the
class, instance and attribute to be accessed. The final word is
the data, in this case the new power level sent to the D8.
Table 3.3 lists and describes the parts of the message body.
Table 3.3 Explicit Message Body
As another example, if you wanted to change the Heat Output
for loop 6, the body of the message would be the same except
that the Instance would be 6 hex.
Reading a Value with an Explicit Message
The logic in Figure 3.11 on page 58 initiates an explicit mes-
sage from the PLC to the D8. This message specifies the Get
Attribute Single service (0E hex) rather than the Set Attribute
Single service (10 hex) used in the previous example.
With the class, instance and attribute specified, the PLC gets
back the current setting for loop 1's Proportional Band. In this
explicit read example you can see not much has changed in the
ladder logic. In fact, the logic could be duplicated from the
previous example with the only change being the contents of
N14:92.
Memory Location Description Example Value Note
N14:50 MSB
Transaction ID
(TXID)
1 hex
Unique number for
message in the queue
N14:50 LSB Command 1 hex
Execute the transmis-
sion block
N14:51 MSB Port 0 hex The DeviceNet port
N14:51 LSB Data Size (in bytes) 8 hex
Size of the message
body: 8 bytes or 4 words
N14:52 MSB Service 10 hex
Get Attribute Single
(See Table 3.21 )
N14:52 LSB MAC ID 1 hex The D8's address
Memory Location Description Example Value Note
N14:53 Class 65 hex Output Object (See Table 3.21)
N14:54 Instance 1 hex Loop 1
N14:55 Attribute 64 hex Heat Output (See Table 3.23)
N14:55 Data 0 hex Sets the Heat Output to 0%