Why does my Sealevel Digital I/O TTL adapter have 5 volts present after system power on?

TTL I/O devices are commonly used to control solid-state relay modules that interface a variety of input and output devices.  An important consideration in these applications is to make sure that the solid-state outputs are properly configured before being energized.

Solid-state relays operate using negative logic, where the off state (logic 0) is 5V. Sealevel TTL devices adhere to the Opto22 standard for solid-state relays and are designed to power up in the off state by having 5V present on all I/O pins at start-up. This prevents unintended activation of connected negative logic devices.

Sealevel TTL I/O adapters follow a specific start-up sequence to provide safe operation by guaranteeing a single transition from no state to either 0V (logic 1) or 5V (logic 0).

  • As soon as power is applied to the TTL adapter, the I/O pins are pulled to 5V through a 10K Ohm resistor. At this point all I/O pins are inputs and power up in a known state.
  • Next, the device driver or firmware applies the preset values from the device configuration to all pins. All TTL I/O pins are still configured as inputs and the preset values have no affect on input pins. Therefore, this step does not change the state of any I/O pins.
  • Finally, the device driver or firmware switches ports that are configured as outputs to an output state. Since the presets were applied before this step, only output pins that are preset to negative logic 1 (0V) change their value. All outputs that are preset to negative logic 0 remain at 5V and do not change state.

All I/O pins remain in this state until the user’s application takes control of the adapter.

In certain specialized applications, you may want your Sealevel TTL device to power on with an off-state of zero volts. To learn more, including how to order modified TTL adapters read the FAQ, Can I guarantee that my Sealevel Digital I/O TTL adapter outputs always power on at zero volts?