1WIO, one DS2408 board for relays, inputs and LED displays

Most 1-Wire devices do one thing: a thermometer measures temperature, a counter counts. The 1WIO was built to be configured. At its center sits a DS2408, the Dallas/Maxim 1-Wire port expander, which connects the bus master to the physical world through eight programmable I/O channels. Around that chip, the board offered three kinds of option:

• four SPDT relay outputs
• four opto-isolated inputs
• up to eight LED displays

A further board option added a DS18S20 temperature sensor, so a 1WIO installed in a utility room or a garage could report the temperature there while it switched loads or watched contacts.

The DS2408 is well suited to this role. Each of its eight channels can serve as an input or as an open-drain output, and the chip keeps an activity latch per channel that records whether a line has changed since it was last read. That means a bus master polling every few seconds can still catch a brief contact closure between polls. Because the DS2408 has its own 64-bit ROM ID with family code 29h, many 1WIO boards can share one bus and be addressed individually.

The 1WIO is discontinued, and Midon Design stopped selling at the end of 2023. The board remains a useful reference design for anyone building DS2408 hardware.

Paired with the TEMP08 1-Wire serial interface, a 1WIO fitted with relays could be used instead of the RELAY05 relay board. The difference is in the wiring. The RELAY05 connected to its host interface through a dedicated extension and switched up to eight DPDT relays from serial commands. A 1WIO sits on the 1-Wire bus like any sensor, so relays can be placed where the loads are, on the same CAT-5 cable that already carries the thermometers.

With the TEMP08 firmware of the time, up to 20 1WIO boards configured as relays could be used on one interface, or up to 10 configured as LED displays. Each relay board provides four SPDT (single pole, double throw) contacts. SPDT relays have a common terminal that connects to either a normally open or a normally closed terminal, so a single relay can switch a load on, switch it off or choose between two circuits.

Typical uses were the ones home automation users still care about: fans and pumps, a garage door opener pulse, a sprinkler valve or an alarm sounder, triggered by rules in the host software. The TEMP08 handled the bus timing and the host only had to send commands over RS232.

With a TEMP08 or a 1WSwitch, the 1WIO connects through a six-wire connector, J9 or J10. In that arrangement power and signals all arrive over the one cable, and the two positions make it easy to run the bus onward to the next device. When the board is used with a different 1-Wire bus master, the pin assignments of those connectors need to be checked carefully, because modular jack wiring was never standardized between makers. An alternative connection to a bus master can be made through connector J12.

Relay coils draw far more current than a 1-Wire chip, so boards with relays fitted need a local supply rather than parasitic power from the data line. Using the supply lines on the six-wire connector avoids running separate power cable to each board.

On current systems the DS2408 is supported directly. OWFS exposes its PIO and sensed channels as files, the Linux kernel has a w1 driver for the chip, and the Home Assistant 1-Wire integration can present its channels as switches and binary sensors when used with an OWFS server. That means an existing 1WIO can still be read and switched by a modern host with a suitable USB 1-Wire adapter.