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Model 8225

Din Rail Multi-Dropped
Short Haul Modem



Reference Manual

0315-0281 Rev C


Printed 03.19.02

Table of Contents

1.0 General Description
2.0 Specifications

3.0 Installation
3.1 Digital Interface
3.2 Analog Interface
3.3 Power

4.0 Operation
5.0 Troubleshooting
6.0 Connector Pin Assignments
7.0 Power
8.0 Help

General Description

The Model 8225 is a short-haul modem designed for full-duplex asynchronous communications over two sets of twisted-pair wires and can be DIN-rail mounted. In addition, the unit incorporates a carrier scheme so that other Model 8225s may be multi-dropped from the same sets of wires.


2.0 Specifications

2.1 Interface

Conforms to RS-232 and CCITT V.24 specifications.

2.2 Connectors

RS-232: DB9F

Analog: Five screw terminals.

Power: 1.3 mm female DC power jack.

2.3 Data Rate

0 to 38.4 kbps

2.4 Transmission

Asynchronous full duplex over four wires (two twisted pairs).

2.5 Transmission Controls

Data Carrier Detect (DCD) turns on after detecting the receive signal from the line. Data Set Ready (DSR) or Data Terminal Ready (DTR) will be a constant high depending the on setting of the DTE/DCE switch. For DTE setting Clear-To-Send (CTS) turns on 40 msec after terminal rises Request-To-Send (RTS).

2.6 Switches


Each modem is equipped with a DTE/DCE switch to allow reversing of the TD and RD signals on the RS-232 interface as well as the proper setup of the control signals.

Line Length

The Model 8225 has a four-position switch is used to set the length of the line the unit will be transmitting over. The following table lists the line length associated with each switch setting.

Switch Position

Line Length 24 AWG


0 10 K


10K 13 K


13K 17 K


17K 20 K



Each unit is equipped with two switches to configure the Model 8225 for multi-drop. The Control switch is used to set the unit for either constant transmission or to use RTS (or CTS depending on the DTE/DCE switch) to enable the transmitter. The Terminator switch is used to enable or disable the receive terminating resistor.

2.7 Indicators

The "Link Status" LED indicates whether the Line Length switch is set properly

2.8 Transmission Range

The Model 8225 will operate from DC to 38.4 kbps over 20 kft of 24 AWG.

2.9 Carrier Control

The carrier is selectable as continuous or switched. In the switched mode, the carrier is controlled by RTS.

2.10 Power

The Model 8225 has been designed to operate from an external DC power source ranging from +15 VDC to +35 VDC at 400 mA.

2.11 Size

3 in W x 4.5 in L x .875 in H (76.2 mm x 114.3 mm x 22.25 mm)

2.12 Environment

-10o to 70o C, 5% to 95% relative humidity, no condensation.


3.0 Installation

3.1 Digital Interface

The digital interface for the Model 8225 is a DB-9 connector.

3.2 Analog Interface

  • Note: The four-wire line between modems must be two twisted pairs. On each modem, the wires marked "T+" and "T-" must be members of the same twisted pair, while "R+" and "R-" must be members of the other.

Screw Terminals: The Model 8225 is supplied with five screw terminals marked "T+," "T-," "R+," "R-," and "GND." When connecting two line drivers together the T+ of one unit goes to the R+ of the other, while T- goes to R-.

3.3 Power

Power to the Model 8225 is applied to the unit through the 1.3 mm power jack located adjacent to the five-position terminal block. A mating 1.3 mm connector is shipped with the Model 8225. The center pin is used for the + connection and the side connection for the ground connection.


4.0 Operation

Connect the Model 8225 to the device that contains the RS-232 port. The DTE/DCE switch on the Model 8225 allows for reversing of the input/output pins of the modem. When the switch is in the DCE position, Pin 2 of the modem is the output from the modem.

Each unit must have its Line Length switch set according to the length of the line that will connect the units together. To verify that the switch is set properly, the Link Status LED should be on.

Switch Position

Line Length 24AWG


0 10K


10K 13K


13K 17K


17K 20K


If the unit is to be used in a multi-drop application then the multi-drop switch must be set to "ON." In addition, the units located at the longest point of the link should have the terminating switch set to IN and all other units must be set to OUT. In a multi-drop application, the equipment that the Model 8225 will be attached to must be able to control RTS. When data is to be transmitted, the equipment must turn RTS on and when the transmission is complete, RTS must be turned off.


5.0 Troubleshooting

The following is a list of solutions to problems that may arise during installation:  

1. The data being received is garbled. 
a) The DTE/DCE switches are not set properly (see section 4.0, "Operation"). 
b) The equipment the Model 8225 is connected to does not have the communication parameters set the same. 
c) T+ and T- are reversed going to R+ and R-. 
d) One of the four wires is broken. 
e) Wires are not paired properly. T+ and T- should be one twisted pair and R+ and R- should be the other. 
f) The Line Length switch is not set properly. 

2. No data is being received. 
a) The customer equipment is not connected to the Model 8225. 
b) The DTE/DCE switches are not set properly (see section 4.0, "Operation").
c) One or more wires between the modems are open. 
d) The link connection exceeds the maximum specified distance.

If the unit is believed to be defective, operation can be verified if one of the devices to which the modems are 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 8225 to the terminal via the RS-232 connector and make the following loopback connections at the analog interface: T+ to R+ and T- to R-

Set the switch indicators to the following positions:

     Set Range Select to "Short"
     Set Term to "In"
     Set Multi-drop to "Off"
     Set DCE/DTE to "DCE"

If the modem is functioning correctly, any data entered on the keyboard should appear on the screen.


6.0 Connector Pin Assignments








Data Carrier Detect




Receive Data




Transmit Data




Data Terminal Ready




Signal Ground




Data Set Ready




Request to Send




Clear to Send



7.0 Power

The Model 8225 can be powered from an external DC power source that can supply a minimum of 15 VDC to 35 VDC at 400 mA. Power is applied to the unit though the 1.3 mm power jack.


8.0 Help

Assistance can be obtained by visiting our Technical Support Center.


TELEBYTE warrants the equipment to be free from defects in material and workmanship, under normal and proper use and in its unmodified condition, for 12 months, starting on the date it is delivered for use. TELEBYTE's sole obligation under this warranty shall be to furnish parts and labor for the repair or replacement of products found by TELEBYTE to be defective in material or workmanship during the warranty period. Warranty repairs will be performed at the point of manufacture. Equipment approved for return for warranty service shall be returned F.O.B. TELEBYTE factory and will be redelivered by TELEBYTE freight prepaid, except for non-continental U.S.A. locations. Non-continental deliveries will be sent COD freight plus import/export charges.

The above warranty is in lieu of all other warranties, expressed or implied, statutory or otherwise, including any implied warranty of merchantability or fitness for a particular purpose. TELEBYTE shall not be liable for any damages sustained by reseller or any other party arising from or relating to any equipment failure, including, but not limited to consequential damages nor shall TELEBYTE have any liability for delays in replacement or repair of equipment.

Out of warranty equipment may be returned to the Greenlawn, NY customer service facility prepaid as described above. Return shipping charges will be billed to the customer. The repaired unit will have a 90-day warranty. In those cases where "NO TROUBLE" is found, a reduced charge will be billed to cover handling, testing and packaging.

Whether in or out of warranty, a Return Material Authorization (RMA) number is necessary. Assistance can be obtained by visiting our Technical Support Center.

Document No. 0315-0281/Rev. C