
number of coincidences, such as sending a status request just as the interface hits a
limit, just as it starts decelerating etc. Any motion control requirement explicitly takes
priority over communications under these circumstances, and can therefore cause a
communications problem unless you wait for echoback. Also different issues of software
may have different execution time characteristics which can make a routine based on
delays work at one software issue, and not another.
Accurate defensive programming is required if safety constraints are to be met and possible
damage to hardware prevented.
Interactive Programming
To help you achieve reliable, trouble free interactive programming between a Controller
Positioner and a PC/PLC the following guidelines will be useful.
• The interface should be operated in terse mode (EX0) to suppress unsolicited error
messages and extended text output. This will also allow for extra communication time.
• The program should include an interface response check, especially during critical
processes such as sequence download. Checks should be made for:
*No response at all
*Incorrect characters
*More or less characters than expected
*Control characters such as EOF formed by errors
*The correct response
Where the correct response is the echoback of the transmitted characters or characters
returned by interrogation commands or a combination of the two.
• The command buffer is 256 characters long. When it has filled up, the interface will
echo a <CTRL>G instead of the correct character. Subsequent characters received will
‘overwrite’ the last character in the buffer until a ‘free’ space occurs.
• Given the above, it is preferable to transmit one character and wait for correct echoback
before transmitting the next character. This enables an error to be rectified
immediately.
• When monitoring the echoback characters in verbose mode (EX1), some are echoed as
a combination of characters, for example:
<CR> is echoed as <CR> <LF>.
This is not the case in terse mode (EX0).
• If there is no check for echoback on each individual character, an explicit delay of at
least 1ms between each character must be included. This may mean sending
characters individually rather than in strings.
• Certain interface commands, notably ON, Z etc. take some time to execute and
consequently momentarily interfere with communications. An explicit delay (several
seconds is typical) should be inserted after these commands before another is sent.
• This interference of communications can involve temporarily not servicing the RS-232
interrupt on the Controller, allowing subsequent characters received by the UART,
during this condition, to cause an overrun error within the UART.
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