Horner APG SmartStack I/O HE800DNS600/HEDNS600 User Manual
Page 85
MAN0577-03-en
DeviceNet Modules
HE800DNM650 & HE800DNS600
CHAPTER 9: EXPLICIT MESSAGING USING DNM650
9.1 General
The DNM650 supports both POLLED and EXPLICIT connections. Explicit Messaging requires a great
deal of overhead in both the DNM650 and the OCS. Multiple PLC scans may be required to access the
required data between the OCS and the DNM650. As a result, Explicit Messages should be reserved for
access to infrequently needed data, such as configuration or tuning parameters only.
The sequence below presents a general description of the process that must be executed to service an
Explicit request.
1. The ladder code builds an explicit request message in a group of %R registers.
2. The ladder code plugs the start of and length of the transmit and receive buffers into four %AQ
registers. The transmit and receive buffers must both be located in the %R register space of the OCS.
3. The ladder code then sets the “Send Explicit Message” command bit.
4. The DNM650 periodically checks the “Send Explicit Message” command bit.
5. If the “Send Explicit Message” command bit is set, processing of the explicit request begins. This
command bit must remain on until success or error status is returned.
6. The four %AQ registers are examined and checked for validity.
7. The DNM650 requests the PLC to send the transmit buffer.
8. The DNM650 then checks the MACID contained in the transmit buffer. Several checks take place,
MACID out-of-range, referenced node not configured and referenced node not On-line.
9. Then the message is formatted and sent to the referenced node.
10. When the response message is received, a check is made to see if the allocated receive buffer is
large enough to accept the response message.
11. The message is then sent to the block of %R registers designated as the receive buffer.
12. The Explicit Transaction Complete bit is then set.
13. If there were any errors detected in any of the previous steps, the process is aborted and the
appropriate error status bit along with the Explicit Transaction Complete bit are set.
9.2
Building Explicit Messages
In the following example, let’s assume that we want to read the polled consumption size from the node at
MACID 3, we also want to locate the transmit buffer at %R101, the receive buffer at %R51 and we will
also allocate 20 bytes to the receive buffer.
BYTE
NUMBER
REGISTER
NUMBER
DESCRIPTION
TRANSMIT
BUFFER DATA
0
R101 LSB
MACID (Node Address)
03
1
R101 MSB
Service Code (Get Attribute Single)
14
2
R102 LSB
Class ID (Connection Class)
05
3 R102
MSB
00
4
R103 LSB
Instance ID (Polled Connection)
02
5 R103
MSB
00
6
R103 LSB
Attribute # (Consumption Size)
07