Maple Systems STEPware-100 - Manual (Page 19)

5-Wire RS-422/485
Host Controller
OIT Terminal Block
(MAP460D, OIT3600)
OIT Port 1
(OIT3100 and
OIT4100 Series)
OIT Port 1
(all other models)
TXD+ Pin 4 — RXD+ Pin 8 — RXD+ Pin 4 — RXD+
TXD- Pin 5 — RXD- Pin 7 — RXD- Pin 1 — RXD-
RXD+ Pin 6 — TXD+ Pin 1 — TXD+ Pin 9 — TXD+
RXD- Pin 7 — TXD- Pin 2 — TXD- Pin 6 — TXD-
signal ground Pin 8 — return Pin 4 — return Pin 5 — return
RS-422/485 (Network mode)
Network mode supports the 3-wire and 5-wire RS-422/485 configurations. The 3-wire
configuration has both transmit and receive on the same pair of wires plus a signal
common. The 5-wire configuration has differential signal pairs for transmit and receive
plus a signal common. In both configurations the signal pairs must be twisted and
surrounded by an overall shield.
5-wire RS-422/485 supports full-duplex communications; which means that the host
controller can transmit data to the OITs and receive data from the OITs at the same time.
3-wire RS-422/485 only supports half-duplex communications; which means that the host
controller cannot transmit and receive data at the same time. Although 3-wire RS-422/485
installations may be less costly than 5-wire installations, the increased complexity in
programming the host controller for 3-wire RS-422/485 may increase development time.
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.
16 STEP1 Protocol Operation Manual
1010-0096, Rev. 04
HOST
OIT #2 OIT #3 OIT #1 LAST OIT
NETWORK CABLE
TERMINATOR