RS485 Internal circuit

Question

I am having a hard time understanding the RS485 internal circuit. The datasheets don't provide any explanation about the internal circuit. One thing I’m struggling to understand is the voltage level at pins A and B. Are these voltages referenced to the supply ground, or is there another reference point/? Additionally, would there be a short circuit if pin A were accidentally connected to ground? How are the voltages at these pins generated? I’m not sure if my question is detailed enough, but I hope this clarifies what I’m trying to ask.

Image source: Renesas - RS-485 Transceiver Tutorial

Answer 1

The internal circuits are not relevant and each chip or manufacturer may implement them in any way they see fit, as long as the chip meets RS-485 specifications or works better that required.

Basically a transmitter drives A and B pins in a H bridge fashion to output few volts with polarity chosen by state on data pin. The receiver then compares if voltage on wire A is above or below voltage of wire B to detect level of data sent.

Yes voltages on A and B are referenced to chip ground.

Yes there will be a short circuit if you short A or B to ground accidentally.

The voltages come from power supply. The power supply and ground pins are not drawn in the block diagram for simplicity, as every IC needs power supplies and ground to work.

Answer 2

Your link is to a tutorial on the subject of RS-485 and the transceivers - but you're then asking questions about how a specific IC might work.

You'll get the information you need by looking at a datasheet for an RS-485 transceiver IC. An industry-commonplace example is the Texas Instruments THVD1410 which has the same internals as your diagram.

The diagram is only part of the detail of the IC, you can't use the diagram in isolation to understand the IC. You need to refer to the pin-out, description text and then to the electrical specifications. Between them, they cover it all.

Answer 3

One thing I’m struggling to understand is the voltage level at pins A and B. Are these voltages referenced to the supply ground

Yes.

What you are asking about is the input common mode voltage range. Wikipedia has a summary of the 485 specification, including a subsection on Signals. The A and B signals are referenced to GND, but can go below GND.

When the standard was written, the vast majority of logic circuits ran on +5 V and GND, so that is the "best case" for the A and B signal voltages. The standard allows for a pair of 0-to-5 V signal lines to ride on a DC offset of up to +/-7 V. Thus, the total allowable input voltage range is from -7 V to +12 V. This means that there can be a difference of up to +/-7 V between two system grounds and the transceiver chips will still operate correctly.

https://en.wikipedia.org/wiki/RS-485#Signals

Answer 4

One thing I’m struggling to understand is the voltage level at pins A and B. Are these voltages referenced to the supply ground, or is there another reference point/?

The A/B voltage levels are usually referenced to the ground pin on the IC. For an isolated RS485 chip it would be referenced to the ground pin on the same side as the A/B pins.

Additionally, would there be a short circuit if pin A were accidentally connected to ground?

Whether there will be a short circuit sort of depends on exactly which chip you are using. Some like the SN65HVD17xx are designed to survive shorts directly to power (up to +70V) or ground. In the case of the SN65HVD17xx the datasheet specifies that the chip has circuitry to limit the current in any shorts to 250mA.

Some chips have no such protection and shorting to power or ground will burn up the chip and the current will only be limited by the resistances of the transistors at the output stage (usually only a few ohms).

How are the voltages at these pins generated? That question is not answerable since each chip can choose its own implementation as long as they meet the RS485 specification with respect to voltage levels, drive strength, impedance, and timing.

Usually there will be some transistors at the output stage on the A/B pins. One to drive the pin high and another to drive it low, plus possibly some circuitry for slew rate limiting or fault protection.

Answer 5

The transmitter part of the tranceiver IC connects voltages to A and B - as explained in other answers, datasheets and application notes. The voltage polarity between A and B presents the state of the transmitted data bit.

Only one transmitter can talk at a time, the others must stay silent. Every appliance connected to the same cable must obey common "whose turn is it to talk"-rules.

To keep up the order the appliances must run properly designed communication software. The software controls tranceiver IC wire DE to connect the transmitter to A and B only when the appliance talks to other appliances. If 2 transmitters were connected at the same time the voltage between the wires couldn't follow neither of the transmitters. It would be a short circuit like connecting different voltage batteries in parallel.

Communication software design is a branch of engineering. It needs easily much more effort than designing properly working circuits. Fortunately programmers can reuse existing well proven software modules just like hardware designers can utilize component application notes.