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10

DATA TERMINATION REQUIREMENTS 
 
RS232 Data Signals 

The RS232 interface standard is a point to point transmission protocol for digital signals. 
It allows for a single transmitter device communicating to a single receiving device. This 
configuration is mirrored in the opposite direction to create one bi-directional data path. 
Devices using this protocol have terminating resistors built into their data path. This 
eliminates the need for any switch selection for termination resistors in the RS232 mode.  

 

 

RS422 Data Signals 

The electrical interface described in RS422 is a data transmission standard for balanced 
digital signals. It allows for a single transmitter device communicating to as many as 32 
receiving devices. This type of data signal is well suited to systems that require data to 
be distributed to several points without a return data path. Several companies offer 
camera telemetry controllers using this data interface. Because there is only one 
transmitting device on the network, this one may remain active at all times. There is no 
need for the driver to go into a high impedance state to allow others to "talk." A 
configuration using multiple drivers on the same wire pair is exclusive to RS485 
described in the next section. In most cases termination resistors are not required to be 
applied to RS422 data inputs or outputs. In long copper runs (over 500 feet) termination 
resistors may be required to eliminate data reflections. Termination resistors for RS422 
data can be selected in the MTX-8485 and MRX-8485 using switch #2 and switch #5.   

 
RS485 Data Signals 

RS485 differs from RS422 in the ability of the transmitter devices to go into a high 
impedance (Hi-Z) state. This allows multiple transmitter devices to reside on the same 
wire pair. The software must dictate a protocol that allows one device to transmit at any 
one time to prevent data crashes. Data wiring can use two wires or four wires. Using two 
wires the system works in half duplex. This means that data is exchanged between two 
points sequentially. When a four-wire system is used, the system may be full duplex. In 
many cases the system head end controller will continuously poll data from all remote 
devices. The remote devices all respond back to the head end (one at a time!) as they 
are addressed. This property of the network rests solely in the hands of the software 
(firmware). 
 
The driver chips used in RS485 communications are capable of changing into their high 
impedance state very rapidly. On even short lengths of wire there can exist a residual 
voltage after a driver circuit turns off. This can interfere with circuits that are used to 
detect the Hi-Z state. It is very important that the copper communications lines be 
terminated with resistors across the data wire pair. The best place to locate such  
resistors is at the furthest electrical devices at the ends of the wire pair. For instance, if 
several RS485 devices are connected in a daisy chain fashion, the wire connection 
would loop across all devices in a chain. The furthest two points in the chain would need 
to be terminated. Termination resistors for RS485 four wire data can be selected in the 
MTX-8485 and MRX-8485 using switch #2 and switch #5. A termination resistor for 
RS485 two wire data can be selected in the MTX-8485 and MRX-8485 using switch #5.   

OFFSET BIAS – RS485 

The RS485 specification requires receivers to detect input signals down to 200mVp-p of 
voltage level. In many cases this can cause systems to be sensitive to noise on the data 
wires. In an effort to eliminate the effects of low levels of noise, some manufacturers of 

Summary of Contents for MRX-8485

Page 1: ...Copyright 2003 American Fibertek Inc 0530JD Instruction Manual MTX 8485 MRX 8485 Four Channel Video Multiplexer with Bi directional Data...

Page 2: ...2 Table of Contents...

Page 3: ...r Source 5 Power Connection 5 Input Output Connections 5 Data Configuration 6 MTX 8485 Status LED Indicators 8 MRX 8485 Status LED Indicators 9 Data Termination Requirements 10 Offset Bias RS485 Data...

Page 4: ...s and multiplexes these signals along with the data signals onto a single optical output port for connection to the fiber transmission system Correspondingly the MRX 8485 receiver converts the optical...

Page 5: ...iber optic connection is made via a ST connector located at the back of the unit Be sure to allow sufficient room for the required minimum bend radius of the fiber cable used Video input and output co...

Page 6: ...RS422 data channel or 3 1 bi directional RS232 data channel or 4 1 bi directional RS232 data channel and 1 bi directional RS422 data channel These units are factory shipped configured for RS485 4 wire...

Page 7: ...hes that identifies the specific switch function and up down direction for RS485 transmission The following is an illustration of the data configuration switches that identifies the specific switch fu...

Page 8: ...ion Protocol DATA 1 TX DATA 1 RX DATA 2 TX DATA 2 RX RS485 Configuration RS485 Tx RS485 Rx Hi Z Hi Z RS422 Configuration RS422 Tx RS422 Rx Not Used Not Used RS232 Configuration Not Used Not Used RS232...

Page 9: ...Tx and Rx LEDs The data channels associated with these front panel DATA LEDs are listed below Configuration Protocol DATA 1 TX DATA 1 RX DATA 2 TX DATA 2 RX RS485 Configuration RS485 Tx RS485 Rx Hi Z...

Page 10: ...s multiple transmitter devices to reside on the same wire pair The software must dictate a protocol that allows one device to transmit at any one time to prevent data crashes Data wiring can use two w...

Page 11: ...will cause communications to fail Offset bias for two wire RS485 can be applied on the MTX 8485 and MRX 8485 by setting switch 4 and switch 6 down Please note that these switches work as a set and mu...

Page 12: ...ent consequential or incidental damages No agreement or understanding expressed or implied except as set forth in this warranty will be binding upon American Fibertek unless in writing signed by a dul...

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