In ACTIVE mode, stack devices can embed their fault status before retransmitting a response frame if DEV_CONF[FCOMM_EN] = 1. When the [FCOMM_EN] = 1, the stack devices repurpose the SOF bit in the response frame’s device address byte, register address bytes (both high and low address bytes) to a fault status bit instead. See Figure 9-42. This will be referred to as fault status bits in the rest of this section.

Figure 9-42 Embed Fault Status in Communication Response Frame

To pass on the fault status of the stack devices, the host sends a broadcast read or sends a single device read to the ToS device. Both types of reads will result in response frames passing through every device in the daisy chain, giving each device an opportunity to OR their fault status to the fault status bits in the response frame.

An example of a response frame going through a daisy chain from a single device read command to the top device is shown in Figure 9-43.

Figure 9-43 Transfer Fault Status in ACTIVE Mode (Respond to a Single Device Read)

When a device has no fault, it will OR the fault status bits with 0b000; otherwise, it will OR the fault status bits with 0b111. Hence, if a fault exists in any device in the daisy chain, the fault status bits will be 0b111. For the base device to assert the NFAULT pin, it requires at least two bits of the fault status bits to be 1.

Additionally, when a device detects a response frame with at least two of the fault status bits being 1, the device will also set the FAULT_COMM3[FCOMM_DET] = 1. If this fault is not masked, the device will be in fault state as well. Next time a response frame is transmitted, this device will OR the fault status bits with 0b111.

Host performs a broadcast read to detect which device in the daisy chain is at fault and what type of fault.