CONTREX CX-1200 - Manual (Page 4)

decimal places value of 2 byte integer
0
0
1 256
2 512
3 768
4 1024
Explicit Messaging:
While the CX1200 DeviceNet option card is capable of explicit messaging, most users do
not use this feature. There are some initializing activity that requires some explicit
messages, but the scanner card (host) will take care of these automatically. The User
does not usually have to deal with these at all. Just follow the setup procedures for your
scanner.
An explicit message can be sent to change the communications time-out values in the
CX1200 card. This is known as the EPR, or Expected Packet Rate. The default time-out
is 10 sec, but can be set to indefinite. However, the easiest solution is to tell the scanner
to "Ping" the CX1200. The scanner will sent a "no-op" message at regular intervals to
keep the node alive if scanning is interrupted for any reason.
Faults and Errors:
An explicit message can query the CX1200 to determine a communication fault, or
reason for a rejected parameter value. Most users do NOT go to that much trouble. The
scanner itself will report many types of communications errors in an easier to use format.
Most users that need to verify that a "write" action was successful will use a write &
read sequence. Just add the write parameter to the read list. Then write the new value,
wait 1 scan, and compare the read value to the original write value.
Remote/Local, or Auto/Manual Selection:
The rejection of a write value may not always be an error. It might be intentional. This
feature can be used to toggle between an Auto Mode (PLC determines setpoint), and a
Manual Mode (CX1200 operator manually enters a setpoint). Of course the operator can
use other means to acquire control over the setpoint (change blocks via a switch), but
there also is a way for the PLC to make that decision and override the operator's setpoint
if needed. The PLC would send a "reasonable" setpoint value any time it wanted to take
control. Since polling happens very rapidly, any setpoint entered by the operator would
be immediately over-written by the value from the PLC. When the PLC wishes to
relinquish control to the operator, it tries to send an illegal or "unreasonable" setpoint
value to the CX1200. Usually you choose a negative number, or very large number, (all
9's). This number must be outside of the allowable min/max for that parameter. The