NanoBoard 3000 - RS-485 Serial Interface

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Modified by Admin on Sep 13, 2017

The NanoBoard 3000 provides an RS-485 serial interface, catering for high-speed, full duplex differential data transmission. An 8-way PC (modular telephone) jack is used to provide a serial port, connection to which can be achieved using standard twisted-pair cabling.

RS-485 serial interface port.

Providing the interface between an appropriate controller within the FPGA design and the physical RS-485 bus itself, is a high-speed ISL8491 RS-485 Transceiver device (from Intersil). This device essentially provides signal translation – from digital to differential voltage (suitable for transmission over the RS-485 bus) and vice-versa.

On the design side, transmit (TXD), receive (RXD) and corresponding enable (TX_EN, RX_EN) signals are wired between the ISL8491 and four pins of the User FPGA device. On the RS-485 bus side, differential signals are wired to the PC jack for both transmission (pins 1(+) and 2(-)) and reception (pins 3(+) and 6(-)).

Table 1 summarizes operation of the ISL8491 when transmitting data – how the TXD and TX_EN inputs affect the TX_P and TX_N differential outputs:

Table 1. Affect of relevant inputs on transmitter differential output.

Inputs		Outputs
TXD	TX_EN	TX_P	TX_N
0	0	High Impedance State
0	1	0	1
1	0	High Impedance State
1	1	1	0

Table 2 summarizes operation of the ISL8491 when receiving data – how the RX_EN input from the FPGA design, and the RX_P and RX_N differential inputs from the RS-485 bus, affect the RXD output sent back to the FPGA design:

Table 2. Affect of relevant inputs on receiver output.

Inputs		Output
RX_EN	RX_P - RX_N	RXD
0	>= +0.2V	1
0	=< -0.2V	0
1	Inputs Open	1
1	X	High Impedance State

The ISL8491 is powered from the motherboard's 5V supply and and can operate at speeds up to 5Mbps. It presents as a single unit load on the RS-485 bus, which in turn can support up to 32 transceivers.

A configurable jumper header – designated J26 – is provided to apply a load of 120Ω across the receiver differential input lines. This resistance is used to prevent signal reflections caused by fast transitions. Insert a jumper on this header when this transceiver is located at either end of the RS-485 bus in a multi-transceiver network.

Location on Board

The 8-way PC jack (designated J23) is located along the top edge on the solder side of the board. Looking from the rear, it can be found to the left of the RS-232 interface port (J17).

RS-485 Port (as seen from the rear of the board).

The ISL8491 device (designated U46) is located on the solder side of the board, to the left of the HIN232A RS232 transceiver device (U59).

ISL8491 device.

The configurable jumper header (designated J26) is located on the component side of the board, in-between the ADC and DAC screw terminals (J13 and J15 respectively).

120Ω termination jumper.

Schematic Reference

The RS-485 interface circuitry can be found on the following sheets of the motherboard schematics:

Sheet 70 (RS485_ISL8491.SchDoc, entitled RS485 Communication IC)

Sheet 71 (CON_RS485_RJ45.SchDoc, entitled RS485 Connector).

Design Interface Component

Table 3 summarizes the available design interface component that can be placed from the FPGA NB3000 Port-Plugin.IntLib, to access and use the RS-485 interface.

Table 3. RS-485 port-plugin component.

Component Symbol	Component Name	Description
	`RS485CNTR`	Place this component to interface to the ISL8491 device and subsequent RS-485 serial port.

Further Device Information

For more information on the ISL8491 device, refer to the associated datasheet (ISL8491.pdf) available at www.intersil.com.