Simplex 4100ES — Negative Ground Fault

The panel prints Negative Ground Fault. This is the only wording the manuals give for the message, and it is listed as a fault of the CPU motherboard configuration. The message concerns the RUI shield connection on the CPU motherboard.

The manuals define the message as a negative ground fault occurring on the RUI — in other words, a ground fault arising from the RUI shield connection on the CPU motherboard. It points to how the RUI shield is set up on the board rather than to a trouble elsewhere on the system.

• Some devices that connect to the RUI have inherently grounded shield terminals. The manual states that for such devices 24 C cannot be used.
• The fault appears when the 24 C setting is used anyway: the RUI SHIELD jumper P9 on the CPU motherboard is placed in position 1-2 while devices with inherently grounded shield terminals are connected to the RUI. The manual says plainly that if 24 C is used in this case, a Negative Ground Fault will occur.

1. Look at what is connected to the RUI. Some RUI devices have inherently grounded shield terminals, and for these the 24 C setting cannot be used.
2. On the CPU motherboard, locate the RUI SHIELD jumper P9 and note its position.
3. If P9 sits in position 1-2 (the 24 C setting) while grounded-shield devices are on the RUI, this is exactly the condition the manual names as the cause of the fault.
4. Reposition P9 so that 24 C is no longer selected, since the manual states that 24 C cannot be used with such devices.

• RUI SHIELD jumper P9 on the CPU motherboard: position 1-2 selects the 24 C setting. With RUI devices whose shield terminals are inherently grounded, 24 C cannot be used and produces the Negative Ground Fault.