RKC INSTRUMENT AE500 - Page 24

3. SETTING FOR COMMUNICATION
IMAE02-E3
18
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RS-485 (2-wire system) send/receive timing
The sending and receiving of RS-485 communication is conducted through two wires; consequently,
the transmission and reception of data requires precise timing. Typical polling and selecting
procedures between the host computer and AE500 are described below:
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Polling procedure
a
: Response send time after AE500 receives [ENQ] + Interval time
b
: Response send time after AE500 sends BCC
c
: Response send time after AE500 receives [ACK] + Interval time or
Response send time after AE500 receives [NAK] + Interval time
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Selecting procedure
a:
Response send time after AE500 receives BCC + Interval time
b:
Response wait time after AE500 sends ACK or Response wait time after AE500 sends NAK
To switch the host computer from transmission to reception, send data must be on line. To
check if data is on line, do not use the host computer’s transmission buffer but confirm it by
the shift register.
Whether the host computer is using either the polling or selecting procedure for
communication, the following processing times are required for AE500 to send data:
-Response wait time after AE500 sends BCC in polling procedure
-Response wait time after AE500 sends ACK or NAK in selecting procedure
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Fail-safe
A transmission error may occur with the transmission line disconnected, shorted or set to the high-
impedance state. In order to prevent the above error, it is recommended that the fail-safe function be
provided on the receiver side of the host computer. The fail-safe function can prevent a framing error
from its occurrence by making the receiver output stable to the MARK (1) when the transmission line
is in the high-impedance state.
Send data
(Possible/Impossible)
Host computer
- - - - -
Possible
Impossible
or
Sending status
AE500
Possible
bca
Impossible
- - - - -
Send data
(Possible/Impossible)
Sending status
N
A
K
A
C
K
E
N
Q
S
T
X
B
C
C
E
O
T
Possible
Impossible
or
Possible
ba
Impossible
- - - - -
Send data
(Possible/Impossible)
Host computer
Sending status
AE500
Send data
(Possible/Impossible)
Sending status
S
T
X
B
C
C
N
A
K
A
C
K