Automating Manufacturing Systems - Process Control and ...

plc network - 28.28
4. Controlnet, Profibus, Ethernet with multiple subnets
5 the maximum transfer rate is 230 Kbps, with 11 bits per byte (1start+8data+2+stop) for 20909
bytes per second. Each memory write packet contains 17 overhead bytes, and as many as 2000
data bytes. Therefore as many as 20909*2000/(2000+17) = 20732 bytes could be transmitted
per second. Note that this is ideal, the actual maximum rates would be actually be a fraction of
this value.
6. The OSI model is just a model, so it can be used to describe parts of systems, and what their
functions are. When used to describe actual networking hardware and software, the parts may
only apply to one or two layers. Some parts may implement all of the layers in the model.
7. When more than one client tries to start talking simultaneously on a bus network they interfere,
this is called a collision. When this occurs they both stop, and will wait a period of time before
starting again. If they both wait different amounts of time the next one to start talking will get
priority, and the other will have to wait. With CSMA/CD the clients wait a random amount of
time. With CSMA/BA the clients wait based upon their network address, so their priority is
related to their network address. Other networking methods prevent collisions by limiting
communications. Master-slave networks require that client do not less talk, unless they are
responding to a request from a master machine. Token passing only permits the holder of the
token to talk.
28.9 ASSIGNMENT PROBLEMS
1. Describe an application for DH networking.
2. The response times of hydraulic switches is being tested in a PLC controlled station. When the
units arrive a ‘part present’ sensor turns on. The part is then clamped in place by turning on a
‘clamp’ output. 1 seconds after clamping, a ‘flow’ output is turned on to start the test. The
response time is the delay between when ‘flow’ is turned on, and the ‘engaged’ input turns on.
When the unit has responded, up to 10 seconds later, the ‘flow’ output is turned off, and the
system is allowed to sit for 5 seconds to discharge before unclamping. The result of the test is
written to one of the memory locations from F8:0 to F8:39, for a total of 40 separate tests.
When 40 tests have been done, the memory block from F8:0 to F8:39 is sent to another PLC
using DH+, and the process starts again. Write the ladder logic to control the station.
3. a) Controls are to be developed for a machine that packages golf tees. Each container will normally
hold 1000 tees filled from three different hoppers, each containing a different color. For
marketing purposes the ratio of colors is changed frequently. To make the controller easy to
reconfigure, the number of tees from each hopper are stored in the memory locations N7:0,
N7:1 and N7:2. The process is activated when an empty package arrives, activating a
PRESENT input. When filling the package, the machine opens a single hopper with a solenoid,
and counts the tees with an optical sensor, until the specified count has been surpassed. It then
repeats the operation with the two other hoppers. When done, it activates a SEAL for 2 seconds