Summary

• RS485 half-duplex line driver
• 1kV galvanic isolation
• ATtiny841 microcontroller
• I²C / SPI / second UART breakout header
• Standard AVR ISP6 header

What is it?

This board contains an isolated RS485 line driver combined with an ATtiny841 microcontroller. It allows the ATtiny chip to communicate with a shared-bus RS485 line while electrically separating it with a 1kV isolation barrier. This lets the RS485 line operate at a different electrical potential - useful if the line has to travel some distance, especially between buildings or if there's a potential for different systems running on different electrical power phases. This is often used in stage equipment that uses DMX-512 for example.

The primary USART of the ATtiny841 is connected directly to the isolated RS-485 line driver, allowing a complete application to be built on the board such as a DMX transmitter or receiver, or other control interface. The ATtiny841 is clocked by a 16.000MHz ceramic resonator, allowing precise clock rates to be generated for serial communication.

All the remaining GPIO pins from the microcontroller that aren't dedicated to the clock or RS485 driver are brought out to the labelled breakout header; these are PA3-PA7 and PB2. Because the ATtiny841's SPI, I²C and secondary USART modules are connected to these pins, this makes it convenient to operate the board either as an SPI or I²C slave or master, as part of a larger project. Alternatively, these lines can be used directly in an application, either as GPIOs, ADC inputs or PWM outputs, in any combination. The microcontroller's RESET pin is also present on the breakout header to use if required. A separate 2x3 header connectors allows the ATtiny841 to be programmed via the usual Atmel ISP method.

Why did you make it?

I've been building a lot of DMX-512-controlled equipment. A DMX receiver is ideally supposed to be electrically isolated from the sender, to avoid electrical issues across stages. It's also useful whenever the line has to travel a long distance; which is typically where RS485 is most used. Additionally, DMX-512 operates at a fast line rate of 250kBaud, which puts quite a tight timing requirement on other work the MCU might be doing at once. By offloading the DMX receiving part of the application into a separate microcontroller, the timing requirements can be carefully handled here away from other things the main controller is doing.

What makes it special?

Unlike most of the other RS485 driver modules you tend to find, this one combines the power and logic isolation elements right there on the board and connected to a dedicated application offload controller, giving a complete RS485 handling in one convenient place.

If you're just looking for an isolated RS485 line driver without an onboard microcontroller, perhaps to attach it to an existing MCU or other async serial-talking device, you might instead be interested in the RS485 Isolator product.