Enable RS-485 Communications on an IEI Tank* AIoT Developer Kit

ID 675495
Updated 1/22/2019
Version Latest



Industrial IoT environments commonly use devices that support the Modbus* serial communications protocol, a popular, reliable, and time-tested architecture. These Modbus-compatible devices, instruments, and sensors connect easily via Ethernet or serial cables. Modbus has many variants, including Modbus-RTU protocol, which uses RS-232, RS-422, or RS-485 signaling. This article describes the BIOS settings and connector pinouts required to configure the ports for RS-485, half-duplex mode.

IEI AIoT Tank Developer Kit Back Panel

Figure 1. IEI Tank* AIoT Developer Kit Back Panel

The IEI Tank* AIoT Developer Kit has two ports, COM 5 and COM 6, that can be enabled for Modbus serial communications, as indicated by Figure 1. 

Modify BIOS Settings

Modifying and saving BIOS settings requires a reboot of the system. 

  1. At system power up, press DEL or F2 to enter the BIOS setup utility. The AMI* BIOS setup utility opens. 
  2. Select the Advanced tab, as shown in Figure 2, and choose F81866 Super IO Configuration. Press Enter.

    AMI BIOS Advanced Tab

    Figure 2. Advanced Tab

  3. Both serial ports 5 and 6 can be enabled as RS-485. Choose one of these serial ports in F81866 Super IO Configuration, shown in Figure 3, and press Enter.

    AMI BIOS F81866 Configuration

    Figure 3. F81866 Super IO Configuration

  4. Enable the serial port, if not already enabled, which will set the port number and IRQ, as shown in Figure 4. Choose Change Settings.

    AMI BIOS Serial Port Configuration

    Figure 4. Serial Port n Configuration

  5. Modify Serial Port Mode by choosing RS485, pictured in Figure 5. Press Enter to choose the serial port mode setting.

    Serial Port Mode Dialogue

    Figure 5. Serial Port Mode

  6. Figure 6 shows a serial port configured in RS-485 mode. Repeat steps three though five for the other serial port if desired. 

    Serial Port Mode RS-485 Chosen

    Figure 6. Serial Port Mode RS-485 Chosen

  7. Press F4 to save settings. The system will reboot.

Configure RS-485 COM Port Wiring

Devices supporting RS-485 use many different methods for describing pin configurations. Some use A and B as signal names while others use +/- notation.

Figure 7 shows three different devices and their notations. For example, the USB serial device on the right indicates that A corresponds to D+ and B corresponds to D-, while device on the left uses a +/- notation.

RS-485 Pin Examples

Figure 7. RS-485 Pin Examples

The IEI Tank* AIoT Developer Kit manual supports 2-wire half-duplex RS-485 and uses the notation TX485# and TX485+, as listed in Table 1.

RS-485 Serial Adapter Settings by Pin for COM5 and COM6
PIN RS-485
1 TXD485#
2 TXD485+
3 --
4 --
5 --
6 --

Table 1. Serial Adapter Pin Configuration for RS-485

Figure 8 is a diagram of a 9-pin serial port adapter wired for RS-485 half-duplex using pins 1 and 2 for the data. Pin 5 is the ground pin. The COM ports on the developer kit are male, requiring a DB-9 adapter. This type of configurable cable does not ship with the IEI Tank* AIoT Developer Kit.

Serial Adapter DB-9 Using Pins 1, 2, and 5

Figure 8. 9-pin Serial Adapter Wired for RS-485 Half-duplex

NOTE: No damage to the kit occurs if pins are connected in the wrong order. If a device is not communicating, swap the data pins as a first step in troubleshooting. 

Write to Linux* Device Addresses

The back panel of the  IEI Tank* AIoT Developer Kit provides several COM ports, numbered one to six. Linux* device addresses start at zero. When writing applications to send data to COM ports, use the Linux address /dev/ttyS4 for the COM5 physical connector and /dev/ttyS5 for COM6.