Model 278 - RS-232 to
Fiber Optic Short Haul Modem


Model 278 - RS-232 to Fiber Optic Short Haul Modem

Reference Manual

Table of Contents

1.0 General Description
2.0 Specifications
3.0 Operation
4.0 Troubleshooting
5.0 Pin Number Assignments
6.0 Power
7.0 Help
Warranty
 

1.0 General Description

The Model 278, Fiber Optic Short Haul Modem will allow two pieces of equipment with RS-232 interfaces to communicate full duplex at data rates up to 115KBPS and pass two control signals all over a fiber optic link. These features result in a high performance communications device that is immune to noise, surges, lightning, RFI and ground loops.

2.0 Specifications

2.1 Interface : Conforms to RS-232 and CCITT V.24 specifications.

2.2 Connectors
2.2.1 RS-232 : DB25M
2.2.2 Fiber : ST connectors

2.3 Data Rate : 0 to 115 KBPS

2.4 Switches : Each modem is equipped with a DTE/DCE switch to allow Reversing the TD and RD signals on the RS-232 interface and a Test switch to aid in troubleshooting.

2.5 Transmission : Asynchronous, full duplex, over 2 fibers

2.6 Distance : 2Kms

2.7 Wavelength : 830 nm

2.8 Power Budget : 12 dB

2.9 Indicators : TD, RD, Control In 1, Control Out 1, Control In 2, Control Out 2, Line Error, Fiber Signal Valid, Remote Test, and Power

2.10 Power : 115 Volts AC 60 Hz (220 Volts AC 50 Hz optional)

2.11 Size : 5"W x 1.5"H x 5.3"D (127mm x 12.7mm x 134.6mm)

2.12 Environment : 0 to 50 C, 5% to 95% relative humidity

3.0 Operation

3.1 Data Transmission
The Model 278 comes equipped with a DTE/DCE switch that allows the user to connect to any type of asynchronous RS-232 device. When configured as a DTE the Model 278 will connect to a DCE device, such as a multiplexor or modem. When configured as a DCE, the Model 278 will connect to a DTE device such as a PC. The following table describes the pin number assignments for the RS-232 interface. An "I" indicates that the signal is an input to the converter on the RS-232 and an "O" indicates that the signal is an output from the converter on the RS-232 side.

 

RS-232 Pins Common Name DCE DTE
2 Transmit Data I O
3 Receive Data O I
4 Request to Send I O
5 Clear to Send O I
6 Data Set Ready O -
8 Data Carrier Detect O I
20 Data Terminal Ready I O

Note : When configured as a DTE, pin 6 DSR, is not supported.


3.2 Handshake Signal
The Model 278 provides the capability of transmitting two handshake signals in each direction. The RTS/CTS and DSR/DCD/DTR bi-directional control signals are transferred in one byte time, or at a rate of 14.4KBPS.

3.3 Remote Test
The Model 278 is equipped with Remote Test switch to help verify proper operation of the unit. When the Remote Test is initiated a unique pattern is transmitted to the far end. When this pattern is received by the remote end it will turn on the Remote LED indicating that the link is valid.

3.4 Indicators
On the front panel of the Model 278 are ten LED indicators. These indicators can be used to observe the status of the Model 278 and the user's equipment. The following is a list of the LED's and their function.
TD - This LED is used to monitor the data coming from the user's equipment. When the DTE/DCE switch is in the DCE position pin 2, TD, will turn this LED on when there is data present. When in the DTE position pin 3, RD, will turn the LED on.
RD - This LED is used to monitor the data being transmitted out of the Model 278. When the DTE/DCE switch is in the DCE position pin 3, RD, will turn this LED on when there is data present. When in the DTE position pin 2, TD, will turn the LED on.
Control In 1 - This LED is used to monitor the first input hardware handshake signal. When The Model 278 is configured as a DCE then this LED will show the status of RTS. When in the DTE mode, this LED indicates the status of CTS.
Control Out 1 - This LED is used to monitor the first output hardware handshake signal. When The Model 278 is configured as a DCE then this LED will show the status of CTS. When in the DTE mode, this LED indicates the status of RTS.
Control In 2 - This LED is used to monitor the second input hardware handshake signal. When The Model 278 is configured as a DCE then this LED will show the status of DTR. When in the DTE mode, this LED indicates the status of DCD.
Control Out 2 - This LED is used to monitor the second output hardware handshake signal. When The Model 278 is configured as a DCE then this LED will show the status of DSR and DCD. When in the DTE mode, this LED indicates the status of DTR.
Link - This LED turns on when there is a valid fiber optic link between the two Model 278's.
Error - This LED will turn on when a data error has occurred between the two Model 278's.
Remote - This LED will indicate when the remote end has been switched into the remote test mode and a valid remote test pattern has been received.
Power - This LED will be on when ever power is present.

4.0 Troubleshooting

The following is a list of possible problems that may arise during the installation:

1. The data being received is garbled.
a) The DTE/DCE switch is not set properly (see section 3.1).
b) The equipment that the Model 278 is connected to does not have the communication parameters set the same.
c) The loss in the fiber optic cable is greater than the power budget of the unit, 12 dB.

2. No data is being received.
a) Customer equipment not connected to the 278.
b) The DTE/DCE switches are not set properly (see section 3.1).
c) Determine if hardware handshaking is required. If your devices can normally communicate with a cable that has only pins 2,3, and 7 then hardware handshaking is not required. If a cable with pins 4,5,6,8,or 20 is required then hardware handshaking is required. If this is the case verify that the equipment that the Model 278 is connected to uses the same handshaking signals that the Model 278 uses, RTS/CTS and DSR/DCD/DTR.
d) One or both of the fibers between the modems are damaged.
e) Link connection exceeds maximum specified distance.

3. Data OK at first then becomes garbled.
a) Verify that hardware handshaking is being used.
Operation can be verified if one of the devices to which the modem is attached is capable of operating in a full duplex mode (such as a terminal or a PC using communications package such as Procomm). Connect the Model 278 to the terminal or PC and place a short fiber optic cable between the T and R connectors on the rear of the unit.
If the modem is functioning correctly, any data entered on the keyboard should appear on the screen.

5.0 Pin Number Assignments

The following table describes the pin number assignments for the RS-232 interface. An "I" indicates that the signal is an input to the converter on the RS-232 and an "O" indicates that the signal is an output from the converter on the RS-232 side.

 

RS-232 Pins Common Name DCE DTE
1 Frame Ground    
2 * Transmit Data I O
3 * Receive Data O I
4 ** Request to Send I O
5 ** Clear to Send O I
6 *** Data Set Ready O -
7 Signal Ground    
8 *** Data Carrier Detect O I
20 *** Data Terminal Ready I O

*, **, *** These signals are reversed using the DTE/DCE selector switch.
Note : When configured as a DTE, DSR is not supported.


6.0 Power

The Model 278 is powered by a small, wall mounted power cube that supplies 12 VDC @ 300 mA. The power supply must be connected to the Model 278, through the 3.5mm connector located on the rear of the unit, prior to plugging the power supply into the wall.


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301-924-7400 EXT 25 / 301-924-7400 EXT 25 / fax 301-924-7403

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