Sending Commands, Chapter 4
Data Communications with the Watlow Series 988 Family
4.5
ANSI X3.28
ANSI X3.28 Protocol Example
This example demonstrates communication between a master device and a remote
device at address 4. Your personal computer must generate the master’s messages.
Establish Communications Link
Master:
4 <ENQ>
(Attempt to link with device 4.)
Remote:
4 <ACK>
(The link is established.)
End Communications Link
Master:
<DLE> <ENQ>
(End data link.)
Remote:
No response.
“=” Command Example
Master:
<STX> = <sp> A2LO <sp> 500 <ETX> (Set A2LO prompt value to 500.)
Remote:
<ACK> (This will be returned once the unit has completed the value change.
Note: The commands IN1, IN2 and CF may take up to 2 seconds to return this character.
Do not send another message until this character is received.)
“?” Command Example
Master:
<STX> ? <sp> A2LO <ETX> (Request the A2LO prompt value.)
Remote:
<ACK> (This will be returned once the device has the response ready. Do
not send the <EOT> until this character has been received.)
or
<NAK> (The command was not understood. Re-send corrected message.)
Master:
<EOT> (The host gives the device permission to respond.)
Remote:
<STX> 500 <ETX> (The device sends back the requested value. Do not
send a response until the <ETX> has been received.)
Master:
<ACK> (The host received the message correctly.)
or
<NAK> (The host did not understand the response.Device will re-send it.)
Remote:
<EOT> (The device returns control to the host. Do not send a new message
until this character has been received.)
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.
Try again later.
• Protocols are not flexible. Outside of the <STX> <ETX> framing only the defined
protocol characters are allowed. Some programming languages add <cr> to the end
of transmissions. This must be disabled.
• End the communications link and re-establish it with <DLE> and <ENQ> only when
changing to a new device at a different address. The master can communicate
repeatedly with a specific device once the initial data link is established.
