RS485 Transceiver
In its most basic form, an RS485 transceiver comprises a four MOSFET H-bridge along with some extra protective components that are not included here for the sake of simplicity.
The MOSFETs are being activated in pairs, causing the VA and VB voltages to fluctuate between local ground and Vcc.
The resulting waveform can be modeled as a positive and negative differential voltage superimposed onto a common-mode offset voltage (VCM), which is typically close to Vcc/2.
However, there are instances where the local ground voltage may not be 0 V (due to ground loops, lengthy cable runs, electromagnetic interference (EMI), switching noise, and floating grounds). In such cases, the common-mode voltage will be:
VCM = (Vcc - VGND) / 2
A shift in the common-mode voltage can lead to communication errors or even damage the transceiver. The RS485 standard requires a transceiver that operates with any common-mode voltage ranging from -7 to 12 V, to accommodate various potential ground shifts. The RS485 transceivers utilized by Roboteq are capable of functioning within that range.
The absolute maximum values at which the product can be damaged or malfunction are detailed in the following table:
Parameter | Measure Point | Min | Max | Units |
RS485 A/B Voltage | Ground to A/B pins | -10 | 10 | VDC |
RS485 Common Mode Voltage | Ground to A/B pins | -7 | 12 | VDC |
Most RS485 transceivers, including the one integrated by Roboteq, feature a GND pin. Connecting the GND of different devices together guarantees that the ground potential remains consistent throughout the system, which helps keep the common mode voltages within an acceptable range.
CAN bus Transceiver
Similar to RS485, the CAN bus transceiver will toggle the CANH and CANL voltage between Vcc and GND, but with the distinction that the signals will not be symmetrical. The CAN bus communicates in two states: the Dominant state, where CANH equals Vcc and CANL is at GND, and the Recessive state, where both CANH and CANL float at approximately 2.5 V (Vcc/2).
In CAN bus systems, a common ground between nodes is not required, which allows ground offsets to occur. As a result, each node may observe different common mode voltages while the differential signal remains valid. The common mode voltage is defined as (CANH + CANL) / 2, measured relative to the receiver’s ground. According to ISO 11898-2 proper operation, the dominant common mode voltage must remain within 1.5 V to 3.5 V, while the recessive common mode voltage must stay within ±12 V of ground.
The below table lists the absolute maximum values at which the product can be damaged or malfunction
Parameter | Measure Point | Min | Max | Units |
CAN High/Low Voltage | Ground to CANH/CANL pins | -12 | 12 | VDC |
CAN Bus Dominant Common Mode Voltage | Ground to CANH/CANL pins | 1.5 | 3.5 | VDC |
CAN Bus Recessive Common Mode Voltage | Ground to CANH/CANL pins | -12 | 12 | VDC |
References
RS-485 Basics: the RS-485 Driver: www.ti.com/lit/ta/ssztcl5/ssztcl5.pdf?ts=1759846561600
A CAN Physical Layer Discussion: https://ww1.microchip.com/downloads/en/AppNotes/00228a.pdf