When FS26 is in Normal Mode, can FS0B_DIAG / FS1B_DIAG detected short to high?

[Allen_863]

I found that if I'm in Debug Mode and haven't executed EXIT_DBG_MODE, FS0B_SNS and FS1B_SNS will be 0 / SENSED LOW. In this case, if I connect TP88 to 5V, I can get a SHORT TO HIGH DETECTED to set FS0B_DIAG = 1, and if I connect TP11 to 5V, I can get a SHORT TO HIGH DETECTED to set FS1B_DIAG = 1. However, if I'm in NORMAL Mode and have released FS0B and FS1B, because FS0B_SNS and FS1B_SNS will be 1 / SENSED HIGH, I cannot perform a check on FS0B_DIAG and FS1B_DIAG. This is because FS0B_DIAG and FS1B_DIAG are initially 1 / SHORT TO HIGH DETECTED, even though I haven't Input 5V to TP88 and TP11. 

When FS_STATES = 10 / Safety Outputs not released, connecting TP88 to 5V triggers a short to high detected and gives FS0B_DIAG = 1. Connecting TP11 to 5V triggers a short to high detected and gives FS1B_DIAG = 1.

When FS_STATES = 11 / Normal, I connected TP88 / TP11 to 5V, but FS0B_DIAG / FS1B_DIAG is always 0 / NO FAILURE.

This means that FS0B_DIAG / FS1B_DIAG detection is not possible when FS_STATES = 11.

[JozefKozon · Accepted solution]

Dear Allen,

this is expected behavior. In DEBUG mode, the FS0B is forced low and cannot be released by design; the watchdog is effectively opened, and the deep failsafe is deactivated. That means the device expects FS0B to be low, so if you drive the pin externally to 5 V (short‑to‑high), the fail‑safe IO diagnostics see a contradiction and flag the short‑to‑high. Same applies for FS1B. 

In Normal mode (FS_STATES=11), once released, FS0B/FS1B are high (pulled up). The device now expects HIGH, so tying the pin to 5 V does not contradict the expected state — therefore no short‑to‑high is detected and FSxB_DIAG stays 0. That’s why your “connect to 5 V” check appears to “do nothing” in Normal mode. Short‑to‑high diagnostics only make sense when the device is driving the pin low (or expects it to be low). 

With Best Regards,

Jozef