Watlow Series 988 Family - Manual (Page 6)

1.2
Introduction to Data Communications, Chapter 1 Data Communications with the Watlow Series 988 Family
By your request PC-1 wants to talk with device "D-2" to change a set point. PC-1
must first identify D-2 on the line and inquire whether D-2 has time to talk. This
electronic knocking on D-2's door is the "connection."
One of three scenarios may occur when PC-1 calls:
1) D-2 answers saying, "This is D-2, go ahead," and PC-1 begins to talk.
2) D-2 answers and says, "I'm too busy to talk now. Wait until I tell you I'm
finished."
3) D-2 does not answer, which indicates a possible system malfunction.
Let's take the best-case scenario. Here is a simple version of what happens: D-2
answers and hears PC-1 say, "Hello, D-2. Do you have time to talk?"
D-2 acknowledges PC-1 with a "D-2 here, go ahead."
PC-1 then sends an ASCII-encoded message instructing D-2 to change a set point
to 1,000°F. (message)
When PC-1 is finished with its message, it says in effect, "That's all, your turn."
D-2 replies, "OK," and carries out the instruction. D-2 then takes the protocol lead,
and tells PC-1, "The new set point is 1,000°F." (message)
PC-1 says, "OK."
D-2 says, "That's all, your turn."
PC-1 then takes the protocol lead and says, "Thank you, that's all."
D-2 hangs up. (disconnect)
That's basically how the connect, message and disconnect protocols work in
Watlow data communications.
The hallway in this example is really a communications bus — a common connec-
tion among a number of separate devices. A communications system with multiple
devices on a common bus is called a multidrop system.
The exact connect-message-disconnect procedure assures that you are talking to
the correct device.
Protocol maintains system integrity by requiring a response to each message. It's
like registered mail — you know that your letter has been received because the
post office sends you a signed receipt.
In Watlow data communications, a dialog will continue successfully as long as the
messages are in the correct form and responses are returned to the protocol
leader. If the operator enters an incorrect message, or interference comes on to the
data line, there will be no response. In that case the operator or the master must
retransmit the message or go to a recovery procedure. If an operator continues to
enter an incorrect message or interference continues on the data line, the system
will halt until the problem is resolved.
Interfaces