Watlow Series 988 Family - Manual (Page 21)

Sending Commands, Chapter 4 Data Communications with the Watlow Series 988 Family 4.3
ANSI X3.28
"=" Command Example
Master: = A2LO 500 (Set the A2LO prompt value to 500.)
Remote: (This will be returned once the device starts processing. The
master must stay off line.)
Remote: (Processing is done. The master may send a new message.
Note: The commands IN1, IN2 and CF may take up to two seconds to return this
character. Do not send another message until this character is received.)
“?” Command Example”
Master: ? A2LO (Request the A2LO prompt value.)
Remote: (The remote is preparing the response. The master must stay
off-line.)
Remote: 500 (The value is returned and the master may send
another message once the is received.)
or
(The message was not understood. The master may send a new
message.)
For maximum communications speed:
• Do not use a typical delay to wait before looking for a response.
• Scan for returned characters until the correct response is received.
• Use a time out to end a session if a correct response is not received in three
seconds.
How to Communicate Using ANSI X3.28
The ANSI X3.28 protocol provides high quality communications by requiring a
response to every message. With a multiple-device or "multidrop" network, this
protocol prevents confusion among the separate devices. Furthermore, if noise
occurs somewhere in the system, no prompt will change because noise cannot
comply with the protocol.
By placing messages inside a protocol envelope, the messages are protected. In
the following examples you'll see how this works.
ANSI X3.28 protocol rules:
• Every remote device must have a unique address.
• Only the master can initiate a communication session, by addressing a specific
remote device.
• Every message must be framed with an (start of transmission) character
and an (end of transmission) character.
• The master must wait for the remote device to respond to every message within a
reasonable period. If no response occurs, retry the connection or pursue error
recovery.
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CAUTION:
Avoid writing <=>
continuously, such
as ramping set
points or repetitive
loops, to the
controller's
EEPROM memory.
Continuous writes
may result in
premature control
failure, system
downtime and
damage to pro-
cesses and equip-
ment.