Optical Scientific Inc
OFS 2000FW User’s Guide
P/N 1910-901-1
Rev. 03/28/13
29
3.13 RS-232 Connections
The OFS Interface Board J2
DB25 serial port (DCE)
Personal Computer
DB-25 serial port (DTE)
Laptop computer
DB-9 serial port
Pin 2:
RX
Pin 3:
TX
Pins 1 and 7:
Return
Pin 2:
TX
Pin 3:
RX
Pin 7:
Return
A DB-25 male to female (PC
side) cable should be used
because it has pin-to-pin straight
connection
Pin 2:
RX
Pin 3:
TX
Pin 5:
Return
A DB-25 male (OFS) to DB-9
female (PC) cable should be
used because it has a crossover
between Pins 2 and 3, and pin 5
of DB-9 is connected to pin 7 of
DB-25.
3.13.1 Limited Distance Modem (LDM) Connections
The LDM option is available for installations where the Control Box must be installed more than 100 feet from
the user’s computer. The LDM converts the RS-232 levels to current loops for noise free, long distance
transmission. Using 24 AWG wire, the LDM will transmit over 3 miles at 9600 baud.
LDMs are used in pairs, one installed in the Control Box and one at the remote computer. The user must
supply a 2 twisted pair cable with shield to connect between the LDMs. As shown in Figure 3.27, the pairs are
installed in a crossed pattern with the TX of one LDM connecting to the RX of the other LDM, and vice versa.
Since OFS is configured as a DCE device and the user’s computer is configured as DTE, the LDM connected
to OFS should be set as DTE and the LDM connected to computer should be set as DCE.
Figure 3.27 LDM Connection
RX +
TX +
TX -
RX -
RX +
TX +
TX -
RX -
GND
NC
CONTROL BOX
LDM WIRING
USER COMPUTER
LDM WIRING
USER SUPPLIED CABLE
2 TWISTED PAIRS WITH SHIELD
