- 11 -
Example: Mark state
T
×
D+ > T
×
D-
Space state
T
×
D+ < T
×
D-
7-2-3. Connection of RS-485
(1) The logical levels of input and output of this apparatus are
basically as follows:
Mark state
-Terminal < +Terminal
(including the state in which communication is not carried
out)
Space state
-Terminal > +Terminal
However, since the impedance of +terminal and -terminal of
this apparatus is high until just before transmission, the above
levels are output just before commencing transmission.
(2) Example of Connection of RS-485
7-2-4. Terminal resistance
As SR73A and SR74A are not provided with terminal resistance,
connect a 510
Ω
resistance to only the last (the furthest from the
host) station.
7-2-5. Control of 3-state output
(1) Since RS-422A and RS-485 employ the multi-drop system,
in order to prevent signals from colliding, transmission
output should be controlled to maintain high impedance
while communication is not being made or signals are being
received.
(2) It should be controlled so that high impedance turns to the
normal state of output immediately before starting
transmission and high impedance is restored immediately
when transmission ends. Because 3-state control is delayed
by about 3 msec. maximum from the transmission of the end
bit of the end character, a delay time of about 4 msec. should
be provided in case transmission is started immediately.
7-3. Setting of communication parameters
There are four communication-related parameters to be set as
shown below. They are set by means of front keys except for
changing the communication mode from remote to local.
[ ] Communication mode
[ ] Communication address
[ ] Communication data format
[ ] Communication rate
[ ] Delay
7-3-1. Communication mode selecting screen
( )
(1) Local Mode ( )
The COM lamp on the front panel goes out to allow the
setting to be changed by the use of the front keys. In
communication, only the change from local to remote and the
read commands are valid.
(2) Remote Mode ( )
The COM lamp on the front panel turns on and the
commands for read and write through communication
become valid.
Front key operation works only to change from remote to
local.
7-3-2. Communication address (
) setting screen
RS-422A and RS-485 allow connection of a plurality of
apparatuses but in fact, communication is carried out between two
apparatuses. Therefore, the address of each apparatus is
designated by a number between 00 and 99 so as to enable the
designated apparatus to respond.
7-3-3. Communication data format (
) selecting screen
Select data format of communication from 7b_E (data bit length
: 7 bits, parity : even, stop bit : 1) or 8b_n (data bit length : 8bits,
parity : none, stop bit : 1)
7-3-4. Communication rate (
) selecting screen
A rate of transmission from the controller to the host is selected
from among 1200, 2400, 4800 and 9600 bps.
7-3-5. Delay (
) setting screen
In the case of RS-485, a minimum delay time for transmission
from receiving a communication command is set. Delay time is a
set value (0~255)
×
0.1 msec. Nevertheless, actual delay time
includes time for processing the command added to the above
delay time.
7-4. Communication protocol
7-4-1. Communication procedure
Communication is carried out for each block and the
communication right alternates between the controller and the
host from block to block.
7-4-2. Explanation of block
(1) The data format in one block is shown below:
: Start character• • • • • @(40H) Shows the start of a
block.
T x D+
T x D-
R x D+
R x D-
SG
SD+
SD-
RD+
RD-
SG
2
3
4
5
1
Shielding wire
terminal No.
SR73A and 74A Controllers
Line converter
FG (Earthing for protection)
(+)
(-)
SG
(+)
(-)
SG
(+)
(-)
SG
2
3
1
2
3
1
Shielding wire
Host computer
FG (Earthing for protection)
terminal No.
SR73A and 74A Controllers 1
terminal No.
SR73A and 74A Controllers 2
Note:
If terminal resistance is connected to two or more,
correct action is not guaranteed.
Host computer
Transmitting command block
Returning response block
Transmitting command block
(Reading)
(Reading)
Returning response block
(Reading)
(Reading)
Controller (SR73A· 74A)
Transmission
Transmission
Response
Response
@
Higher
position
Lower
position
Higher
position
Lower
position
Text
:
CR
1
2
3
4
5
6
Terminal resistance (510
Ω
)
