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
Содержание MRX-8485
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