The wiring depends on the bus. With SPI, share clock and data lines but give each peripheral its own chip-select. With I²C, share SDA and SCL and assign every device a unique address. With RS-485, share a differential pair and use a protocol such as Modbus RTU to address one node at a time.
Master, slave and shared-bus basics
A master—also called a controller—starts transactions and commonly generates the clock. A slave, peripheral or target responds only when selected or addressed. Several devices can share conductors, but only the intended device may actively drive a shared return path at any instant.
The selection mechanism is bus-dependent: a physical chip-select line in SPI, an address in I²C, or a node identifier in Modbus. The term “master/slave” remains common in datasheets and searches; newer documentation may use controller/peripheral or controller/target.
SPI: one controller with several peripherals
SPI is usually the best fit for nearby devices when throughput and full-duplex transfers matter. It has no universal packet, register or acknowledgement format; each peripheral datasheet defines its command bytes, framing and timing. See TI’s overview at TI SPI overview.
#1 Best Overall
- FT4232HL 4 Channel USB to Serial Adapter Module is a Full Speed (12Mbits/Second) and USB 2.0 High Speed (480Mb/s) to UART/MPSSE Module, this device features 4 UARTs, support UART SPI I2C JTAG RS485 RS422 RS232 to USB
- Entire USB protocol handled on the chip, No USB specific firmware programming required. USB to RS232/RS422/RS485 UART Transfer Data Rate up to 12Mbaud. (RS232 Data Rate limited by external level shifter).
- 2 Multi-Protocol Synchronous Serial Engine (MPSSE) on channel A and channel B, to simplify synchronous serial protocol (USB to JTAG, I2C, SPI or bit-bang) design.
- Low operating and USB suspend current, Configurable I/O drive strength (4,8,12 or 16mA) and slew rate, Supports bus powered, self powered and high power bus powered USB configurations.
- Suitable for USB Smart Card Readers, USB Instrumentation, USB Industrial Control, USB MP3 Player Interface, USB FLASH Card Reader / Writers, Set Top Box PC - USB interface, USB Digital Camera Interface, USB Bar Code Readers
Independent chip-select wiring
+------------------ Peripheral 1
| CS1 -------/
|
Controller +------------------ Peripheral 2
SCLK ------------------------------ SCLK
MOSI/COPI -------------------------- SDI
MISO/CIPO <------------------------- SDO
|
+------------------ Peripheral 3
CS3 -------/
| Controller signal | Peripheral 1 | Peripheral 2 | Peripheral 3 |
|---|---|---|---|
| SCLK | SCLK | SCLK | SCLK |
| MOSI/COPI | SDI | SDI | SDI |
| MISO/CIPO | SDO | SDO | SDO |
| CS1 | CS input | inactive | inactive |
| CS2 | inactive | CS input | inactive |
| CS3 | inactive | inactive | CS input |
MOSI/MISO are also called COPI/CIPO (controller out/peripheral in and controller in/peripheral out). All peripherals share SCLK and the controller-to-peripheral data line. A directly selected design normally needs one additional CS output for each peripheral, as described in TI’s multi-device SPI note.
Safe SPI transaction sequence
- Load that device’s clock polarity, clock phase, bit order, word length and maximum clock settings.
- Make every other CS inactive. Use pull resistors so CS lines cannot float during reset.
- Assert the selected CS.
- Send the device-specific command, register address and data; provide dummy bytes when its read protocol requires them.
- Observe setup, hold and inter-transfer delays.
- Deassert CS, then restore settings before selecting a peripheral with different requirements.
Only one CS should normally be active. The unselected devices must place MISO/CIPO in high impedance; verify this in each datasheet rather than assuming it. A device that keeps driving MISO needs a tri-state buffer, bus switch, separate controller or dedicated return path.
SPI modes and timing differences
Peripherals can require different CPOL/CPHA modes, bit order, idle levels, clock limits or CS polarity. Some require CS to stay low for an entire command-and-response frame; others reset their serial state machine when CS rises. NXP notes that controller settings may need to change between slaves: NXP SPI introduction.
Rank #2
- Working voltage: 3.3V-5V
- Working environment temperature: -40~85degree
- Supports bus-powered, self-powered and high-power bus-powered USB configurations +1.8V (chip core) and +3.3VI/O interface (+5V withstand voltage)
- Support USB bulk data transfer mode (512-byte packets in high-speed mode) UART transfer data rate up to 12 megabaud (baud). (PS232 data rate limit is determined by external level shifter), UART interface supports 7 or 8 bit data bits, 1 or 2 stop bits and odd/even/marker/space/no parity
- Use external EEPROM to save operating mode configuration and USB description string configuration data via USB interface
Daisy-chain SPI
Controller MOSI -> Peripheral 1 input Peripheral 1 output -> Peripheral 2 input Peripheral 2 output -> Peripheral 3 input Peripheral 3 output -> Controller MISO Controller CS ---------------- all peripherals Controller SCLK -------------- all peripherals
A supported daisy chain saves CS pins, but every transfer shifts through every device. Frames become chain-length and bit-order dependent, a failed or unpowered device can interrupt the whole chain, and software must exchange one combined frame. It is not interchangeable with independent-CS wiring.
Outdated Drivers Are Slowing You Down
One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWindows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallWhen there are not enough CS pins
- GPIO expander: Provides more outputs over I²C or SPI, but adds initialization, timing and reset-state dependencies.
- Decoder/demultiplexer: A 2-to-4 or 3-to-8 decoder reduces GPIO use; check enable behavior, polarity and propagation delay.
- SPI multiplexer or bus switch: Isolates non-tristating, powered-off or high-capacitance segments. TI describes these uses at TI SPI switch and multiplexer guidance.
- Second SPI controller: Useful when devices need incompatible electrical or timing conditions.
- Software CS: Ordinary GPIOs often give the most predictable control, especially when CS must transition between words.
Independent-CS pseudocode
void spi_read_device(uint8_t device, uint8_t *command, size_t command_len,
uint8_t *rx, size_t rx_len)
{
deselect_all_devices();
spi_set_mode(cfg[device].cpol, cfg[device].cpha);
spi_set_bit_order(cfg[device].bit_order);
spi_set_clock(cfg[device].max_clock_hz);
cs_low(device);
spi_write(command, command_len);
spi_read(rx, rx_len);
cs_high(device);
}
Add the delays, status polling, checksum or CRC, dummy bytes and timeout recovery required by the specific peripheral.
I²C: shared wires with addressed peripherals
I²C normally uses only SDA (bidirectional data) and SCL (clock). Outputs are open-drain or open-collector, so pull-up resistors create the high level and multiple devices can safely pull a line low. The NXP/TI copy of UM10204 defines addressing, pull-ups, clock stretching and nominal bus modes: I²C-bus specification and user manual.
Rank #3
- The SC16IS752 Serial Adapter is an I2C bus/SPI bus interface to a dual-channel high performance UART offering data rates up to 5 Mbit/s, low operating and sleeping current; it also provides the application with 8 additional programmable I/O pins
- This SC16IS752 chip enables seamless protocol conversion from I2C-bus/SPI to RS-232/RS-485 and is fully bidirectional.
- The SC16IS752 supports SPI clock speeds up to 4 Mbit/s, and the SC16IS752 internal register set is backward compatible with the widely used and widely popular 16C450. This allows the software to be easily written or ported from another platform.
- Dual full-duplex UART, I2C-bus or SPI interface selectable, 3.3 V or 2.5 V operation, Auto hardware flow control using RTS/CTS, Up to eight programmable I/O pins, Automatic RS-485 support (automatic slave address detection)
- SC16IS752 Serial Adapter Module also provides additional advanced features such as auto hardware and software flow control, automatic RS-485 support and software reset. This allows the software to reset the UART at any moment, independent of the hardware reset signal.
The controller sends a target address and read/write bit; only the addressed device acknowledges and participates. UM10204 lists Standard-mode up to 100 kbit/s, Fast-mode up to 400 kbit/s, Fast-mode Plus up to 1 Mbit/s and High-speed mode up to 3.4 Mbit/s. These are protocol-mode limits, not guarantees for a particular chip, cable or layout.
Address conflicts and electrical limits
- Every active device needs a non-conflicting address. Two identical sensors with fixed addresses cannot be separated by changing a software byte.
- Use an alternate address pin, I²C multiplexer, address translator or separate controller bus when addresses collide. A multiplexer creates isolated downstream channels; a translator changes the address seen on one segment.
- Choose pull-ups for the total bus capacitance and required rise time. Device count and cable length are limited by capacitance, leakage and pull-up current.
- Some devices stretch SCL by holding it low. The controller and driver must tolerate that behavior.
I²C read example
bool i2c_read(uint8_t address, uint8_t reg, uint8_t *buffer, size_t length)
{
if (!i2c_start_write(address)) return false;
if (!i2c_write_byte(reg)) return false;
if (!i2c_restart_read(address)) return false;
for (size_t i = 0; i < length; ++i)
buffer[i] = i2c_read_byte(i == length - 1); // NACK final byte
i2c_stop();
return true;
}
SDKs differ in address representation, repeated-start support and timeout handling. A bus held low may require disabling the peripheral, manually toggling SCL, issuing a STOP-like sequence, reinitializing and resetting the offending device; use the MCU and device vendor’s prescribed recovery sequence.
RS-485 and Modbus RTU
RS-485 is an electrical differential signaling layer, not a complete master/slave protocol. UART supplies bytes, while Modbus RTU supplies addressing, function codes, CRC and request/response rules. Other RS-485 systems may use DMX or proprietary framing.
Rank #4
- The CH341A USB Multi Converter enables seamless USB to communication, supporting UART, I2C for connectivity with 3.3V/5V and full USB 2.0 compatibility.
- This USB adapter features configurable data bits (5 8) and synchronous 2/4 wire interfaces, while its standard USB A male connector ensures broad device compatibility without driver modification.
- Ideal for electronics enthusiasts, embedded developers, and professional requiring upgrades or hardware debugging tools.
- Perfect for industrial automation setups, microcontroller programming stations, and projects demanding USB conversion with flexible adaptation.
- Engineered with housing, this USB to UART converter delivers stable data transmission across I2C's four speed modes and SPI interfaces, streamlining platform communication tasks.
Master transceiver
|
======== differential pair ========
| | |
Node 1 Node 2 Node 3
unique ID unique ID unique ID
The controller polls one node, waits for its response, then polls the next. Only the addressed node should enable its driver. Follow the selected transceiver and Modbus guide for termination at the cable ends, biasing, cable topology, driver-enable timing and turnaround delays. Do not assume a universal node count; loading, data rate, cable and termination determine the limit.
Choosing the bus
| Requirement | SPI | I²C | RS-485/Modbus |
|---|---|---|---|
| Short-board throughput | Strong | Moderate | Implementation-dependent |
| Fewest shared signal wires | Needs CS lines | Two | Differential pair |
| Full duplex | Yes | Not typical | Usually half-duplex |
| Long cable runs | Poor without extra hardware | Limited | Strong |
| Addressed multidrop | Physical CS | Built-in addresses | Protocol node IDs |
| Standardized packet format | Device-specific | Bus rules standardized | Modbus provides one common option |
Choose SPI for fast, nearby peripherals and sufficient CS resources; I²C for many low- to moderate-speed ICs sharing two PCB traces; and RS-485/Modbus for physically separated or industrial nodes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Worked design patterns
MCU with three SPI devices
Route SCLK, MOSI/COPI and MISO/CIPO to all three devices. Connect CS1, CS2 and CS3 to separate GPIOs, add inactive-state pulls, store a configuration record for each peripheral, and assert exactly one CS per transaction. Check every MISO output in the datasheets.
Best Value
- The FT2232H USB to Serial Adapter Single chip USB to dual serial / parallel ports with a variety of configurations, Entire USB protocol handled on the chip. No USB specific firmware programming required.
- USB 2.0 High Speed (480Mbits/Second) and Full Speed (12Mbits/Second) compatible.
- Dual Multi-Protocol Synchronous Serial Engine (MPSSE) to simplify synchronous serial protocol (USB to JTAG, I2C, SPI or bit-bang) design, Dual independent UART or FIFO or MPSSE ports.
- Independent Baud rate generators, RS232/RS422/RS485 UART Transfer Data Rate up to 12Mbaud. (RS232 Data Rate limited by external level shifter), USB to parallel FIFO transfer data rate up to 8 Mbyte/Sec.
- Single channel synchronous FIFO mode for transfers upto 40 Mbytes/Sec, CPU-style FIFO interface mode simplifies CPU interface design, MCU host bus emulation mode configuration option, Fast Opto-Isolated serial interface option.
Four I²C sensors with a duplicate address
Connect SDA and SCL to all sensors with correctly sized pull-ups. Give each sensor a distinct address where possible. If two remain identical, place one group behind an I²C multiplexer or on a separate bus; do not attempt to solve the conflict in software alone.
Several Modbus RTU nodes
Connect properly terminated RS-485 wiring, assign unique node IDs, configure identical serial settings, and poll sequentially. Control each transceiver’s driver-enable pin so only the master or the addressed responder transmits.
Quick Recap
Troubleshooting checklist
No device responds
- Confirm ground reference, voltage levels, power rails and connector pinout.
- Check CS polarity and reset-state pulls for SPI, or address and pull-ups for I²C.
- For Modbus, verify baud rate, parity, termination, node ID and CRC.
The first device works but adding a second fails
- Look for overlapping CS assertions or incorrect CS wiring.
- Check MISO contention and whether the second device needs a different CPOL/CPHA, clock limit or CS delay.
- Inspect trace loading, ringing and powered-off devices clamping shared lines.
SPI data is shifted or corrupted
- Verify mode, bit order, word length and CS duration.
- Ensure DMA does not deassert CS before the final clock edge.
- Reduce clock rate and inspect edges with an oscilloscope when decoding alone is misleading.
I²C is stuck low
- Determine whether a slave is stretching SCL or holding SDA after an interrupted transfer.
- Check shorts, pull-up values and voltage compatibility.
- Apply the controller-and-device-specific bus recovery procedure.
RS-485 shows collisions or echoes
- Check driver-enable timing so only one transmitter is active.
- Verify end termination, biasing, polarity and a single defined cable topology.
- Confirm that responses are sent only by the addressed node.
Design checklist
- Identify the actual bus and its electrical voltage levels.
- Draw shared versus dedicated signals and calculate CS or address resources.
- Confirm selection, address uniqueness and reset-state behavior.
- Record per-device timing, mode, clock, pull-up and termination requirements.
- Plan error detection, timeouts, retries and recovery for stuck or powered-off devices.
- Use a logic analyzer for protocol sequencing and an oscilloscope for signal integrity.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
