| Voltage | Communication State | Meaning | Acceptable Range 1 |
|---|---|---|---|
| Bias Voltage | Either Idle or Active Communication | The + to COM Reading, - to COM or + to -. Typically used such as + is biased to com, -, etc. It is the voltage applied to the bus by the EOL termination on the controller to establish a properly biased bus. | See notes below for acceptable ranges based on active or idle communication states. |
| Idle Voltage | Idle | The term idle voltage could be referring to + to COM, - to COM or the + to – (differential) Reading. What it really means is no communication is occurring at the time of the reading. | Typically near 2.7 (+ to com) -2.3 VDC (- to com) with a differential (+ to -) of 0.4VDC. Can vary with wire length & bus load but should not be lower than ~0.2VDC differential or higher than ~0.9VDC differential. |
| Idle Differential Voltage | Idle | This is typically referring to the + to – voltage reading without communication. | ~0.2 to ~0.9 VDC |
| Communication Voltage | Active | The term communication voltage could be referring to + to COM, - to COM or the + to – Reading. Communication is occurring at the time of the reading. | See Bit Voltage and Differential Voltage below. |
| Bit Voltage | Active | + or – with respect to COM when communication is occurring. | +2.0 to +4.0 VDC (+ to Com), -2.0 to -4.0 VDC (- to Com). When comparing the absolute value of these two readings, there should be no more than approximately a 0.2 VDC difference. |
| Differential Voltage, Total Voltage | Active | The + to COM Reading plus the absolute value of the - to COM Reading when communication is occurring. | 2.5VDC to 8VDC is acceptable. |
1 These voltages should typically not be read with a Volt-Ohm-Meter (VOM). Use the FIT instead. Idle bus voltages are read with a VOM when communication is not occurring. When FIT is connected to a bus and then powered on, it will display Offline if the bus is Idle.