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Q: How do I connect “3-Wire” RS-232 using FOSTC
Fiber Converters?
A: A “3-Wire”
RS-232 supports TD, RD and GND coonections. The RS-232 connection
on the FOSTC is wired DCE like a modem. From a computer DB25M
(male) RS-232 port, the cable is straight pin to pin. The
DB9M (male) connector used on the orignal PC/AT changed pin
numbers, but TD still connects to TD, RD to RD and GND to
GND (see left side of the figure below). If the serial port
on the RS-232 device is wired DCE like a modem, then a crossover
of TD & RD between the device and the FOSTC is needed
(see fthe right side of the figure below) Some RS-232 devices
require hardware handshaking, but the the FOSTC and FOSTCDR
don’t support those signals, the connections are looped
back. If needed, use models 232FLST or 9PFLST which support
RTS/CTS connections. Check FAQ for those models.
Connection figure below: Set switch Sw 1:6 to OFF. Other
switches have no effect on RS-232 operation. If using one
end as RS-232, the other end as RS-422 or RS-485, the RS-232
side will match either the left or right side connections
in Figure 1.
Figure 1: RS-232 Connections
1. The RS-232 device on the left side has DTE pinouts which
match a computer with DB9M connector. Pin #3 is output (Tx),
pin #2 is input (Rx), pin #5 is ground. See DTE figure for
DB-25 pinouts. Signal flow for DTE and DCE is opposite.
2 The RS-232 device on the right side has DCE pinouts similar
to a modem, so pin #3 on the device is input (Tx), pin #2
is output (Rx), #5 is ground, note crossover. See DCE figure
for DB-25 pinouts.
How to identify which line should connect to Pin#2 (Input)
from the RS-232 device:
Use a DC voltmeter to measure from the ground wire to each
of the other two RS-232 wires, while the RS-232 device is
powered up. Usually one lead has a minus (-) DC voltage, typically
between –11.5 volts and –3 volts. Whichever lead
has a minus voltage is the lead to connect to our Pin#2. The
other lead usually has nothing or noise relative to ground.
If neither lead has a minus (or positive) voltage on it relative
to ground, recheck for OPEN cable connections to the RS-232
device or the device pinouts. If the device can be configured
multiple ways, make sure all the jumpers and such are set
to RS-232. If you can make the device send data, set the meter
to AC mode and see if from the GND lead to the other two leads,
one shows a AC voltage jump on the meter during data. That
lead is the output to connect to our input. Voltage will be
less than 5 volts.
RS-232 testers which show the active signal lines, Low or
High can be used instead of a meter. 9PMTT for DB9, or 232BOB1
for DB25. Connect first to one side, check light on TD or
RD, then disconnect that side, connect to other side, if the
same light is lighted, a crossover is needed.
Rule of Thumb for RS-232 Cable Connectios – If the
device connects to a computer with a straight through cable,
the device is DCE, and will need a crossover cable to connect
to the FOSTC. If it connects using a Crossover cable (null
modem type) then the device is wired DTE, and it should connect
to our FOSTC directly with a straight through cable.
Figure 2: DTE to DCE Connections
Figure 3: DCE to DCE Connections
This converter provides RS-232 loopbacks internally, but
they are only needed if the software or devices require them
to operate. If the output is looped back and active, the matching
input will be active. The figures show straight wires for
RTS/CTS and DTR/DSR, but if crossed, work the same. On a host
computer, software may choose to ignore them (Hardware Flow
Control: None) if not used or may use them only to verify
a cable is connected. Connections to a computer can use a
standard Modem Cable (not Null Modem) such as the DB9F to
DB25M, 232CAM (6 feet) or 232CAMS (6 inches). Connections
to a Modem device require a crossover such as the 232DTE with
adapter cable or custom wiring.
We Keep The Light On
The FOSTC (and FOSTCDR) keep the light in the fiber turned
on when no data is transmitted and the input signal is in
the MARK state (idle). If light is lost or too low, the electrical
signals go to the SPACE state. The input signal turns the
light Off/On in step with the data. This model has no indicator
for Transmit or Receive, if no light is received, the RD output
will be positive relative to GND (normally negative), and
in RS-422 or RS-485 mode, no light will set the TD(A)- line
high relative to TD(B)+. The usual voltage with light in the
fiber and no signal sets the B line high relative to A (about
4.4 Volts DC no termination). To check, connect a fiber patch
cable from the TX connector into the RX connector.
RS-485 Connections
Q: How do I connect “2-Wire” RS-485 devices with
FOSTC Fiber Converters
to isolate and extend the Bus?
A: See Figure 4 for “2-Wire” RS-485.
Figure 4: 2-Wire RS-485 Connections
In 2-Wire RS-485 mode, the receiver and transmitter must
be connected together, and internal switches #7 and # 8 set
to On. Set the baud rate switches to match the baud rate of
the data so that data is not lost. Not all RS-485 devices
are marked correctly for Data (+) and (-), so if the slave
devices don’t respond after going through the fiber
converters, try swapping the polority of wire pairs to each
converter. When there is no light received from the fiber
in, the TD(B)+ line will be Low relative to the TD(A)-. You
can interconnect or loop back the transmit fiber on one unit
to verfy that the state of TD(A) and TD(B) change with/without
the fiber. Normal bias on RD(B) to RD(A) before termination
is about 3.8 to 4.0 VDC, Data (+) to (-). See our RS-422/485
Application note for more information on biasing. Fiber data
has Priority.
Figure 5: “4-Wire”RS-485 Connection
Set switch #7 to On for RS-485 transmit, to Off for RS-422
mode transmit. Set switch #8 to Off to enable the receiver.
A faint red light can be seen in the transmit ST connector
when the fiber is removed.
Figure 6: “4-Wire”RS-422 Connections
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