MAX485 RS-485 module: specs and pinout
A half-duplex RS-485/RS-422 transceiver with one driver and one receiver. It turns a microcontroller's TTL-level serial signals into a differential A/B pair for long cable runs. This page covers the MAX485 chip; the module board is not covered by an official document.
Specs
| Spec | Value | Source |
|---|---|---|
| Supply voltage VCC | 4.75 V to 5.25 V (single 5 V supply) | source Pin Description table (p.7); General Description (p.1) |
| Duplex | Half duplex | source Selection Table (p.1) |
| Maximum data rate | 2.5 Mbps (driver slew rate not limited) | source Selection Table (p.1); Switching Characteristics (p.3) |
| Transmitters on bus | 32 | source Selection Table (p.1) |
| No-load supply current | 500 uA typ, 900 uA max with DE = VCC; 300 uA typ, 500 uA max with DE = 0 V | source DC Electrical Characteristics, continued (p.3) |
| Control input levels (DE, DI, RE) | VIH 2.0 V min; VIL 0.8 V max | source DC Electrical Characteristics (p.2) |
| Receiver output high voltage | 3.5 V min at IO = -4 mA; receiver output low 0.4 V max at 4 mA | source DC Electrical Characteristics (p.2) |
| Receiver threshold | RO high if A - B > 200 mV, low if A - B < -200 mV; input hysteresis 70 mV typ | source Pin Description (p.7); DC Electrical Characteristics (p.2) |
| Receiver fail-safe | Receiver output is high when the input is open circuit | source General Description (p.1) |
| Low-power shutdown | No (MAX485 has none) | source Selection Table (p.1) |
Power
| Value | Source | |
|---|---|---|
| Supply | VCC 4.75 V to 5.25 V | source DC Electrical Characteristics, continued (p.3) |
| Current | 300 uA typ with driver disabled, 500 uA typ with driver enabled (no load) | source DC Electrical Characteristics, continued (p.3) |
Chip pins
These are the pins of the chip on the module, from the chip datasheet. The module's own header may label or expose them differently.
Rows are the chip's 8-pin DIP/SO pins. The module board (header labels and order, any termination or bias resistors, whether DE and RE are tied together) is not covered by an official document, so check the printed labels on your board.
| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| RO | Receiver output: high if A > B by 200 mV, low if A < B by 200 mV | DIP/SO pin 1 | source Pin Description (p.7) | |
| RE | Receiver output enable, active low; RO is high impedance when RE is high | VIH 2.0 V min, VIL 0.8 V max | DIP/SO pin 2 | source Pin Description (p.7); DC Electrical Characteristics (p.2) |
| DE | Driver output enable; driver outputs are active when DE is high and high impedance when low | VIH 2.0 V min, VIL 0.8 V max | DIP/SO pin 3 | source Pin Description (p.7); DC Electrical Characteristics (p.2) |
| DI | Driver input: a low forces the low driver output, a high forces the high one | VIH 2.0 V min, VIL 0.8 V max | DIP/SO pin 4 | source Pin Description (p.7) |
| GND | Ground | DIP/SO pin 5 | source Pin Description (p.7) | |
| A | Noninverting receiver input and noninverting driver output | DIP/SO pin 6 (bus line) | source Pin Description (p.7) | |
| B | Inverting receiver input and inverting driver output | DIP/SO pin 7 (bus line) | source Pin Description (p.7) | |
| VCC | Positive supply | 4.75-5.25 V | DIP/SO pin 8 | source Pin Description (p.7) |
Build with it
Project ideas for the MAX485 RS-485 module. Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.
- Two-board RS-485 link: Uno boards use 5 V logic, matching the chip's 5 V supply and its 3.5 V receiver-high level; one module sends and the other receives, with DE and RE switched in firmware.
- Multi-node sensor bus: The datasheet allows 32 transmitters on one bus, so a few nodes on a twisted pair are well inside that limit; give each node an address in software.
- Remote status display: The 5 V Uno powers both the LCD backpack and the transceiver from its 5 V rail; the transceiver draws 300 uA typ with the driver off.
Common mistakes
- Connecting the receiver output RO straight to a 3.3 V-only GPIO: the chip runs from 5 V and RO is guaranteed only 3.5 V min when high, so use a level shifter or divider for 3.3 V boards (source)
- Powering the chip from 3.3 V: the MAX485 supply is 4.75-5.25 V; the datasheet lists separate 3.3 V parts as different devices (source)
- Leaving DE and RE unmanaged: driver outputs are active only while DE is high and RO is enabled only while RE is low, so firmware must switch them between sending and receiving (source)
Not covered by the official source
- Module board is not covered by an official document: header labels, termination or bias resistors and any DE/RE tie are unknown.
- Datasheet is Rev 8 (10/03), mirror-hosted copy; a newer revision may exist.
- Absolute maximum ratings and ESD figures are not given on this page.
- Many modules use clone chips; check the marking on your module against this datasheet.
Sources
- link (Maxim MAX481/MAX483/MAX485/MAX487-MAX491/MAX1487 datasheet, 19-0122 Rev 8, 10/03)