Designing RS-485 Links and NetworksR4 protect the receiver and ensure that the input remains biased even if the
wires short together. In a network, only the two end nodes have termination
resistor R5, but each node has its own set of resistors R1–R4.
This termination reduces the noise margin because R3 and R4 each drop a few
tenths of a volt. If the network has fewer than 32 unit loads, you can increase
the noise margin slightly by reducing the values of R1–R4. Multiply each value
by half the total number of unit loads in the network. To calculate the resistor
values in ohms, use:
R1 = NumberOfUnitLoads * 1100
R2 = R1
R3 = NumberOfUnitLoads * 55
R4 = R3
For example, with just two unit-load nodes, R1 and R2 would be 2.2k and R3
and R4 would be 110 Ω.& 5
+
On some half-duplex lines, including those with one control signal for the
driver and receiver enable, the receiver is disabled at times, causing the receiver’s
TTL output to be undefined. To ensure that the output remains high, add a
10k pull-up resistor from the output to +5V. You don’t need the pull up if the
output connects to a microcontroller pin with an internal pull up or if the
receiver drives an input to a MAX232 or other RS-232 interface chip with
internal pull ups.'# -$
TIA-485-A doesn’t recommend a specific cable type, but twisted-pair cable is
inexpensive and performs well in RS-485 circuits. A twisted pair consists of two
insulated conductors that spiral around each other in a double helix (Figure
7-12). The pairs typically have one or two twists per inch. Catalogs may list this
type of cable as network cable or alarm wire. In a twisted pair, much of the
noise that couples into the wires cancels out.
Another option is triaxial cable, which is like coaxial cable except with two con-
ductors rather than one surrounded by a shield. Triaxial cable is expensive, how-
ever, compared to twisted pair.
Thus, here is guideline #5 for RS-485:
Use twisted-pair cable.