American-fibertek MRX-8485C User Manual download pdf (Page 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 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-8485C and MRX-8485C 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-8485C and MRX-8485C using switch #2 and switch #5. A

termination resistor for RS485 two wire data can be selected in the MTX-8485C and MRX-

8485C using switch #5.

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American-fibertek MRX-8485C User Manual Page 10