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The computer then assumes master status again and three options are available:
1.
Repeat Parameter Facility (NAK). If the computer transmits a (NAK) after the valid reply, it causes the 620 to
repeat the parameter that was just received. This allows continuous monitoring of the same parameter without
having to re-establish the connection.
2.
Scroll Mode Facility (ACK). If the computer transmits an (ACK) after a valid 'reply', it causes the 620 to fetch
the next parameter from the parameter list. This facility enables the computer to sequence continuously
through all the parameters of the 620.
3.
Terminate Communication (EOT). The termination procedure is entered when the selection of a particular
instrument is no longer required or when a 620 does not respond to a message or replies with an (EOT)
character. The computer assumes Master status and transmits an (EOT) character to enable all the instruments
on the data link to respond to the next GID-UID address parameter.
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Under certain circumstances the computer may not receive a response from the 620. This could be due to any of the
following reasons:
1.
Group/Unit address identifiers not recognised.
2.
An error (e.g. parity) is found in one or more of the characters up to and including ENQ.
3.
Communications loop failure, perhaps due to noise or wrong baud rate being selected.
4.
Hardware failure.
In these cases the computer should be programmed to "time-out", i.e. wait for a response for a short time (160ms
minimum) before trying again.
The sequence diagram for the data read function is given in Figure 5.24.
INITIAL
ENTRY
E
O
T
ADD
CMD
E
N
Q
RE
ENTRY
NO
REPLY
E
O
T
S
T
X
ADD
CMD
E
T
X
BCC
(READ SAME)
(READ NEXT
PARAMETER)
N
A
K
A
C
K
FETCHES
NEXT
PARAMETER
FROM
LIST
E
O
T
RE
ENTRY
PHASE (A)
PHASE (B)
PHASE (C)
MESSAGE
TRANSFER
TERMINATION
ESTABLISH
CONNECTION
PROTOCOL
PROCEDURE
SENDER
SUPERVISOR
STATUS
STATUS
SLAVE
MASTER
SUPERVISORY
SLAVE
SLAVE
SLAVE
SLAVE
MASTER
MASTER
MASTER
MASTER
SUPERVISOR
584
Figure 5.24 - Reading Data From The 620
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