Black Box MED100A, Fiber Optic Modem Fiber Optic Connections, Dip-SwitchSetup, Typical Setups

MED100A 304 pg. 2 / 4 6132-r1

Fiber Optic Connections

The MED100A uses a separate LED emitter and photo-detector operating at 820 nm wavelength. Connections to the emitter and detector are on ST type connectors. Most multimode glass fiber size can be used including 50/125 μm, 62.5/125 μm, 100/140 μm, and 200 μm. One fiber is required for each connection between a transmitter and receiver. In a point to point configuration, two fibers are required between the two modems, one for data in each direction. A multi-drop ring configuration requires one fiber between TX and RX around the loop. See Figure 2 for typical point-to-point and multi- drop configurations.

The most important consideration in planning the fiber optic link is the “power budget” of the fiber modem. This value represents the amount of loss in dB that can be present in the link between the two modems before the units fail to perform properly. This value includes line attenuation as well as connector loss. For the MED100A the typical connector to connector power budget is 12.1 dB. Because 62.5/125 μm cable typically has a line attenuation of 3 dB per Km at 820 nm, the 12.1 dB power budget translates into 2.5 miles. This assumes no extra connectors or splices in the link. Each extra connection would typically add 0.5 dB of loss, reducing the possible distance by 166 m (547 ft.). The actual loss should be measured before assuming distances.

Figure 2: Typical Setups

Point to Point

RS-232 RS-422

MED100A

TX

Duplex

RX

MED100A

RS-232 RS-422

or RS-485 Device or System

FOSTCDR

SW1:6=OFF

SW1:6 = OFF

RX

Multimode Fiber

TX

FOSTCDR SW1:6=OFF

SW1:6 = OFF

or RS-485

Device or System

Full Duplex

RS-232

RS-422 or RS-485

Device or System

TX

MED100AFOSTCDR SW1:6=OFF= OFF RX

MASTER

Multi-Drop Ring

Multimode

Fiber

SLAVE

TX

RX

TX

RX

TX

RX

SLAVE

SLAVE

SW1:6=ON

MED100A

SW1:6=ON ON

MED100A

SW1:6=ON SW1:6=ON

MED100A

= ONSW1:6

FOSTCDR

=SW1:6

FOSTCDR

FOSTCDR

Dip-Switch Setup

The Dip-Switch (SW1) on the MED100A defines the mode of operation when being used for RS-422 or RS-

485.Positions 1 through 5 on the switch determine the timeout of the RS-485 driver. Because the driver is controlled by hardware, a specific time must be set to tell the hardware how long to wait for data on the fiber side before turning off the RS-422/485 driver. If this time is set too short, the driver could be disabled before transmission is complete, resulting in data corruption. If the time is set too long, the RS-485 device may respond before the RS-422/485 driver in the MED100A is disabled, corrupting this response. We recommend that the timeout be set for approximately one character time or longer. The character times for several different baud rates are selectable on switch positions 1 through 5. If you need a different timeout than what is provided, R10 can be removed and replaced with a different value R9. Table 1 shows different timeout values for the switch positions as well as typical R9 replacement values.

RS-232	RS-232	RS-232
RS-422	RS-422	RS-422
or RS-485	or RS-485	or RS-485
Device	Device	Device
or System	or System	or System

Table 1: RS-485 Timeout Selection

Baud Rate	Pos. 1	Pos. 2	Pos. 3	Pos. 4	Pos. 5	R9	Time(ms)
1200	ON	OFF	OFF	OFF	OFF	Ω	8.20
						820 K
2400	ON	OFF	OFF	OFF	OFF	Ω	4.30
						430 K
4800	OFF	OFF	OFF	OFF	ON	Not Used	2.20
9600	OFF	OFF	OFF	ON	OFF	Not Used	1.30
19.2K	OFF	OFF	ON	OFF	OFF	Not Used	0.56
38.4K	OFF	ON	OFF	OFF	OFF	Not Used	0.27
57.6K	ON	OFF	OFF	OFF	OFF	Not Used	0.22
76.8K	ON	OFF	ON	ON	OFF	Not Used	0.14
115.2K	ON	ON	ON	OFF	OFF	Not Used	0.10
153.6K	ON	OFF	OFF	OFF	OFF	Ω	0.06
						6.2 K
230.4K	ON	OFF	OFF	OFF	OFF	Ω	0.04
						4.3 K
460.8K	ON	OFF	OFF	OFF	OFF	Ω	0.02
						2.2 K

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