Eurotherm 590SP, Product Manual

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590SP Digital Product Manual App. H - 11
Appendix H RS422 Communications (Ports P1 & P2)
H
No Response
Under certain circumstances, the computer may not receive a response from the 590SP. This could be due to any of
the following reasons:
i. Group/Unit address identifiers are not recognized.
ii. An error (for example parity) is found in one or more of the characters up to and including (ENQ).
iii. Communications loop failure perhaps due to noise or selecting the wrong baud rate.
iv. Hardware failure.
v. Parameter SERIAL LINKS::MAIN PORT (P1)::SRL LINK ENABLE is disabled.
In these cases the computer should be programmed to “time-out”, that is, wait for a response for a short time (160
msec minimum) before trying again.
Sending information from the computer
Establish Connection
Connection is established when a 590SP sends:
(EOT) (GID) (GID) (UID) (UID)
followed immediately by the data transfer
(STX) (C1) (C2) (D1) (D2) (D3).....(DN) (ETX) (BCC)
Note: The data transfer message is identical to that transmitted by a 590SP when giving a valid reply.
The symbols of this message are defined as follows:
(STX) Start of text character
(C1)(C2) Parameter specified by mnemonic
(D1 to DN) Parameter value
(ETX) End of text character
(BCC) Block Check Character - verification check digit which is the exclusive OR of (C1) to (ETX) inclusive
and must be calculated by the computer before transmission.
Responses
After completing the transmission, the 590SP responds to it by sending (ACK), (NAK), or by giving no reply.
i. Positive acknowledgment (ACK)
After the 590SP receives the message, it performs the following tasks:
a. Checks for any parity errors in the message.
b. Verifies that the (BCC) character corresponds to the data pattern received.
c. Verifies that the (C1), (C2) command characters are a valid mnemonic that may be written to.
d. Verifies that the data (D1 to DN) is valid and not out-of-range.
e. Updates the parameter with the new value contained in the message.
Only when all these tasks have been successfully completed does the 590SP send the (ACK) response to the
computer. This signifies that the message was correctly received and implemented. If any of the checks fail,
the 590SP immediately sends a negative acknowledgment.