System Board 6328

MAXREFDES69#: Isolated RS-485 Communications Micro PLC card

The MAXREFDES69# is a dual-channel isolated RS-485/RS-422 micro-PLC reference design. Built in an industrial form factor, the design makes use of Maxim Integrated’s MAX14855 isolated RS-485/RS-422 transceivers with integrated transformer drivers for isolated data and power technology. On-board connectors allow the RS-485/RS-422 signals to be connected to an external bus and the on-board USB connector can be used to interface with the STM32 microcontroller. The MAXREFDES69# can be powered from a mico-PLC backplane or from the USB connector, simplifying the setup for fast and easy evaluation.

The board features a micro USB connector for quick connection to a PC for evaluation. No power supply is required as the board derives power from the USB connection. Refer to the Details tab for more information and performance data. As with all Maxim reference designs, hardware design files and firmware can be downloaded from the Design Resources tab. The board is available for purchase.


  • High-speed isolated RS-485 communication
  • Isolated power and data on isolated transceivers
  • Micro-PLC form factor
  • Extended protection with TVS on RS-485 lines
  • Device drivers
  • Example C source code
  • Test data


  • Industrial control and automation
  • Process control
  • PLC


Industry 4.01 marks the fourth industrial revolution, characterized by distributed, intelligent control systems. Breaking from a past with large, centralized programmable-logic controllers, Industry 4.0 allows for highly configurable, highly modular factories, which accept an ever-increasing number of sensor inputs, while operating at a higher output than ever before. The ultra small PLC, or micro PLC, lies at the heart of the Industry 4.0 factory, providing high performance with ultra-low power consumption, in an ultra-small package. MAXREFDES69# is Maxim’s micro-PLC isolated RS-485 communications card. Hardware and firmware design files as well as results of lab measurements are provided.

The MAXREFDES69# features two full duplex-isolated RS-485 transceivers. Each receiver input has a selectable 120Ω termination resistor. The MAXREFDES69# design integrates two full-duplex isolated RS-485 transceivers (MAX14855); a STM32F4 microcontroller; an FTDI USB-UART bridge; and a regulated +3.3V power rail (MAX17515). The entire system typically operates at less than 500mW and fits into a space roughly the size of a credit card. While targeted for an industrial, micro-PLC application, the MAXREFDES69# may be used in any application that requires high RS-485 data rates (up to 25Mbps) and ESD protection.

Figure 1. The MAXREFDES69# reference design block diagram.

Detailed Description of Hardware

The power requirement is shown in Table 1.

Table 1. Power Requirement for the MAXREFDES62# Reference Design

Power Type Input Voltage (V) Input Current (mA,typ)
On-board power, STM32, FTDI, Isolated RS-485 transceivers 5 20

MAXREFDES69# uses the MAX17515 (U100) step-down DC-DC converter to convert the +5V supply from the USB to +3.3V to power the STM32 (U200) microcontroller, the FTDI (U400) USB-UART bridge, and the MAX14855 isolated RS-485 transceivers (U300, U301).

The MAX14855 transceivers (U300, U301) provide both data and power isolation between the SMT32 microcontroller (U200) and the RS-485 network. The combined power and data isolation achieved is 2.75kV.

Detailed Description of Firmware

The MAXREFDES69# uses the on-board STM32F103 microcontroller to communicate with the RS-485 transceivers. The user transmits and receives data through the RS-485 transceivers through a terminal program. The simple process flow is shown in Figure 2. The firmware is written in C using the Keil µVision5 tool.

Figure 2. The MAXREFDES69# firmware flowchart.

The firmware accepts commands and transmits characters or text files through the MAX14855 (U300) Y and Z driver outputs. Connected in loopback, the transmitted data is received through the A and B receiver inputs on the MAX14855 (U301) and compared to the original data that was sent. The complete source code is provided to speed up customer development. Code documentation can be found in the corresponding firmware platform files.

Quick Start

Required Equipment:

  • Windows® PC with a USB port
  • MAXREFDES69# Board


The reference design is fully assembled and tested. Follow the steps below to verify board operation:

  1. The MAXREFDES69# utilizes the FTDI USB-UART bridge IC. If the Windows cannot automatically install the driver for the FTDI USB-UART bridge IC, the driver is available for download from
  2. Connect the USB cable from the PC to the MAXREFDES69# board.
  3. Open Hyperterminal or similar Terminal program on the PC. Find the appropriate COM port, usually a higher number port, such as COM4, or COM6, and configure the connection for 921600, n, 8, 1, none (flow control).
  4. The MAXREFDES69# software displays a menu (Figure 3).
  5. For immediate signal testing, configure the RS-485 transceivers in loopback mode as follows: use a wire to connect the Y terminal of the P300 terminal block to the A terminal of the P301 terminal block. Use a wire to connect the Z terminal of the P300 terminal block to the B terminal of the P301 terminal block.
  6. Press 0 in the terminal program to start the Keypress Loopback Test.
  7. Enter a character.
  8. Verify that the character received is the same as the character sent.

Terminal program main menu.Figure 3. Terminal program main menu.

Lab Measurements

Equipment used:

  • Windows PC
  • Oscilloscope
  • MAXREFDES69# board
  • 10m cat5e cable

Output ripple at 4A load.Figure 4. Loopback functionality (text file transfer); cable = 0m. Termination enabled on both transceivers (CH3 = VDDB at U300, CH4 = A - B).

Figure 5. Loopback functionality (text file transfer); cable = 10m. Termination enabled on both transceivers (CH3 = VDDB at U300, CH4 = A - B).

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1. The new generation of manufacturing production is called Industry 4.0 in Germany and Smart Manufacturing System elsewhere. See, Securing the future of German manufacturing industry, Recommendations for implementing the strategic initiative INDUSTRIE 4.0, Final report of the Industrie 4.0 Working Group, Industry 4.0 Working Group, Acatech National Academy of Science and Engineering, April 2013, Henceforth cited as Industrie 4.0. Although the Industrie 4.0 report is focused on Germany, the implications of the German research and findings are recognized for industry in other countries. See also Ferber, Stefan, “Industry 4.0 – Germany takes the first steps toward the next industrial revolution,” Bosch Software Group, Blogging the Internet of Things, October 16, 2013,

There are many sources for Smart Manufacturing Leadership. An interesting summary report of issues and topics can be found at the Smart Manufacturing Leadership Coalition Committee Working Meeting, Minneapolis, MN, U.S., Thursday, October 20, 2011, . Also see, Implementing 21st Century Smart Manufacturing, Workshop Summary Report, Smart Manufacturing Leadership Coalition, June 24, 2011, A simple web search on the topic will reveal considerably more references.

Quick Start

Required equipment:

  • Windows® PC with a USB port
  • MAXREFDES69# Board