RS485 bus voltage?

Question

Does anyone know the voltage situation regarding multi-drop nodes on an RS485 bus please?

Nick Gammons RS485 article shows 2 different hardware configurations, one using 5v devices, and another using 3.3v devices, and I am fairly sure my Pelco PTZ CCTV system uses 12v.

But I have never seen anything mentioned anywhere about needing to voltage-match nodes on an RS485 bus.

So I am unsure whether the MAX driver chips isolate the RS485 bus voltage from the mcu logic levels.

Or do any lower voltage devices (3.3v or 5v) need logic-level matching to their MAX driver chips, which all need powering from the higher bus level (perhaps 12v)?

Basically, I wish to add an 'intelligent' arduino controller to an existing ptz cctv system.

Answer 1

RS-485 is a differential bus. As long as the difference in voltage between the two lines reaches certain thresholds the signal can be interpreted unambiguously regardless of the system supply voltages. Isolation (from ground loops) is not provided by default by the driver, but can be implemented easily enough.

Answer 2

According to my datasheet for the LTC1480:

• –7V to 12V Common Mode Range Permits ±7V Ground Difference Between Devices on the Data Line

Absolute maximum ratings

• Supply Voltage (V CC ) : 7V
• Driver Input Voltage : – 0.3V to V CC + 0.3V
• Driver Output Voltage : ±14V
• Receiver Input Voltage : ±14V
• Receiver Output Voltage : – 0.3V to V CC + 0.3V

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I am fairly sure my Pelco PTZ CCTV system uses 12v.

Judging by the above you would need to drop Vcc on the part involving your 12V gadget and use level shifters for the logic levels (if you used this particular chip).

Answer 3

I suspect that the line-driver Vcc may be using a commercial standard of 5v which we are not aware off.

The RS485 Vcc VMax is only 7v anyway, so presumably the thousands of different 12v RS485 CCTV devices on the market must use line-drivers chips with a Vcc of 5v because they were historically assumed to be interfacing to TTL logic levels, and therefore still will be for compatibility.

So for integration, I think any 3.3v logic nodes will require level-matching up to a line-driver chip which has its Vcc connected to 5v for RS485 bus compatibility.

Line-driver Vcc would not normally be a problem for any home-brew systems, but could offer a potentially serious gotcha if mixing 3.3v home-brew with existing commercial 5v equipment.

I had been expecting it to be an already known problem, but apparently not, so it would seem to be something worth being aware of.