B&B Electronics MAP450D - Manual (Page 11)

There are several different forms of communications networks. Maple Systems’
OITs utilize the multidrop format. The host controller, located at one of the network
ends, is connected to the OITs via a continuous com-link. Each OIT is connected to
the network by locally tapping into the com-link with short stub cables. The end of
the com-link must be properly terminated to reduce noise pickup and interference.
The total length of the com-link, from the host controller to the network cable
terminator, can be up to 4,000 feet.
If you decide to use 3-wire RS-422/485, the following limitations apply:
• The host controller must disable or tri-state its RS-485 transmitter when it is not
sending data to the OITs on the network. To simplify programming, the host
should disable or tri-state its RS-485 receiver whenever transmitting.
• The ability to stay logged on to a particular OIT by sending the command terminator
instead of the line terminator is not available in this mode. Therefore, each command
sent from the host controller should end with the appropriate line terminator.
• If the host controller sends a command that requires a response from the OIT, the
host controller must wait for a response from the OIT before sending another
command. Otherwise, a situation can occur in which the OIT and the host
controller attempt to transmit at the same time cuasing unpredictable results.
• The ASCII strings sent by the OIT’s function keys should not include any
control characters which the other OITs on the network could misinterpret, such
as the line terminator or command terminator.
INSTALLATION MANUAL 11
1010-0046A, REV 00
HOST
Less than
4000'
TO OIT
Less than 12"
Shielded Cable
OIT #1
OIT #100
TERMINA
TOR
TERMINA
TOR
To More
OITs
OIT #2
From Host or
Previous OIT
See
Detail A & B
Detail B
5 Wire Termination
Detail A
5 Wire Termination
To More
OITs
From Host or
Previous OIT
CHASSI
S
GND
RXD+/
TXD+
RXD-
/T
XD-
SI
G
GND
SI
G
GND
CHASSI
S
GND RXD- SI
G
GND
TXD+ TXD-
TO OIT
CHASSI
S
GND
TXD+ TXD- RXD+ RXD-
SI
G
GND
SI
G
GND
RXD+
Drain
Wires
2 Pair Twisted
Shielded Cable
24 Gage Wire
3 Pair Twisted
Shielded Cable
24 Gage Wire
Drain
Wires