Specifications

RV-M7 Technical Manual
16
If you wish to run the RS-485 in full-duplex mode using two M7s, the interface must be wired with two
separate pairs of wires; A&B on one pair, and Out+ & Out- on the other pair.
The M7’s RS-485 I/O circuit uses 3.3V logic to drive the lines, and the RS-485 pins are ESD protected to
±15kV and 7kV human body model. Input current is less than 150uA. Output current when not driving
the line is less than 50uA.
Bias and Termination Resistors
RS-485 installations typically have a termination resistor across the A and B lines. For low-speed
operation (<57600 baud), this resistor is probably not necessary. If you wish to use a terminating
resistor, a value of 150 ohms should work in most applications.
These A/B pin names are all in use on various types of equipment. The RS-485 signaling specification
states that signal A is the inverting or '-' pin and signal B is the non-inverting or '+' pin. The same naming
is specified in the NMEA standards.
When an RS-485 network is in an idle state, all nodes are in listen (receive) mode. Under this
condition there are no active drivers on the network. All drivers are tri-stated. Without anything
driving the network, the state of the A and B line is unknown. If the voltage level at the receiver's A
and B inputs is less than ±200mV the logic level at the output of the receivers will be the value of the
last bit received. In order to maintain the proper idle voltage state, bias resistors must be applied to
force the data lines to the idle condition.
M7 RS-485 receivers have built-in bias resistors, offering the network a true “1/8 unit load”. In most
allocations, these internal bias resistors are sufficient, and an external bias resistor is not required.