Maple Systems STEPware-100 - Manual (Page 22)

3-Wire RS-422/485 (Network mode)
If you decide to use 3-wire RS-422/485, the following limitations apply:
1. 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.
2. 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.
3. 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 causing unpredictable results.
4. 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.
Network Termination
The com-link cable must be properly terminated to reduce the amount of interference and
noise pickup from the surrounding environment. This is accomplished by using a
termination circuit at both ends of the network cable. The following figure shows a
typical termination circuit. The host termination is only needed if the host controller’s
RS-422/485 adapter does not contain any built-in terminator circuitry. The end
termination must be done at the extreme end of the com-link cable.
Getting Started 19
1010-0096, Rev. 04
5
RXD+
Return
4
DE9P
RXD- 1
9
TXD-
TXD+
6
Shield
OIT
From or To
Host or OIT
CHASSIS GND
RXD+/TXD+
RXD-/TXD-
SIG GND
SIG GND
5
COM
RXD+
RXD-
+24V
3
4
2
1
TB1
Shield
OIT
From or To
Host or OIT
+24V
GND
Power Supply
RTN 8
*
*
RTN and COM are internally connected in OIT. *
7
TXD+
TXD- 6
CHASSIS GND
RXD+/TXD+
RXD-/TXD-
SIG GND
SIG GND
Shield
OIT
From or To
Host or OIT
CHASSIS GND
RXD+/TXD+
RXD-/TXD-
SIG GND
SIG GND 4
RXD+
Return
8
TXD+ 1
7
TXD-
RXD-
2
RJ45
3-wire RS422/485 (OIT3100 and OIT4100 Series, except OIT3160/4160)