Maxim / TI / many · Integrated circuit

MAX485 RS-485 Transceiver

Half-duplex RS-485 driver and receiver, the usual way to get Modbus or a long serial run off a microcontroller.

1.Pinout diagram

Pins 1-4

MAX485

Pins 5-8

Pin detail

Hover a pin for its mappings and gotchas. Click to keep it here.

Fig. 1. MAX485 RS-485 Transceiver, PDIP-8. Hover or tap a signal to light up every pin that carries it.

Half duplex: only one device may drive the bus at a time. DE high enables the driver, RE low enables the receiver. Tie them together and drive both from one GPIO.
Release DE as soon as the last stop bit is on the wire, not when the UART buffer empties. Letting DE linger is the single most common RS-485 bug, and it makes the bus look dead to every other node.
A is the non-inverting line and B is the inverting one. Swapping them gives you clean idle levels and nothing but framing errors, so it is worth labeling the terminal block.
Terminate with 120 ohm across A and B at each end of the cable, and only at the ends. Mid-run terminators load the bus.
Idle bias matters: with every driver off the bus floats and receivers see noise as start bits. Add a pull-up on A and a pull-down on B, around 680 ohm, somewhere on the segment.
The plain MAX485 is a full unit load, so 32 nodes. MAX487 and friends are quarter unit load and reach 128.
This chip is 5 V. Driving its DI from 3.3 V logic works, but RO comes back at 5 V and needs a divider into a 3.3 V micro. The MAX3485 avoids the whole problem.

What has to go around the chip before it will run.

Fig. 2. Half-duplex RS-485 node, drawn from Table 1. Dashed parts are optional. Crossing wires are not connected.

Table 1. External parts, and why each one is there.
Part	Between	Why
120 Ohmoptional	A (6) to B (7)	Termination, and only at the two physical ends of the cable. Mid-run terminators load the bus.
680 Ohmoptional	A (6) to VCC	Idle bias, so receivers see a defined state when no driver is enabled.
680 Ohmoptional	B (7) to GND	The other half of the idle bias pair.
Wire link	RE (2) to DE (3)	Ties receive enable to drive enable so one GPIO controls direction.
100 nF	VCC (8) to GND	Supply decoupling at the pin.

Ground must be common across the bus as well as the differential pair. A and B alone work on the bench and fail over a long run.

Every pin, alias and mapping. This is the table that prints.

Table 2. Pin assignments.

Complete pin reference for MAX485 RS-485 Transceiver
Pin	Name	Also called	Functions	Notes
1	ROUART	R, RXD	Receiver output	Goes to your microcontroller's RX.
2	RE	/RE, nRE	Receiver enable, active low	Usually tied to DE and driven by one GPIO.
3	DE	-	Driver enable, active high	High to transmit. Drop it the moment the last stop bit is out.
4	DIUART	D, TXD	Driver input	Goes to your microcontroller's TX.
5	GND	-	Ground	The bus needs a common ground reference as well as the differential pair.
6	A	D+, non-inverting	Bus line A	Non-inverting. Idles high relative to B.
7	B	D-, inverting	Bus line B	Inverting.
8	VCC4.75-5.25 V	-	Positive supply	100 nF at the pin.

Found a wrong pin? The whole board is one file, data/boards/max485.json.