American Fibertek MRX-986-UTP-H - Manual (Page 7)

7
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." 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
on data inputs to eliminate data reflections.
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.
OFFSET BIAS – RS485
The RS485 specification requires receivers to detect input signals down to 200mVp-p of voltage
level. In some 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 equipment that
communicate using RS485 have introduced a small voltage bias to the data lines. This is
usually accomplished using a 470 Ohm resistance to +5V on the positive line and 470 Ohm
resistance to ground on the negative line. When used in conjunction with the appropriate
termination resistors referred to in the previous section, this introduces about a 300 mV offset,
improving noise immunity.