Common RISC OS 3 POST failure errors

When powering on a RISC OS 3 based machine, it goes through a series of basic tests to ensure the machine is operating correctly. This is known as a Power On Self Test sequence or POST sequence for short.

There are many different RISC OS 3 POST errors and there are a handful of errors that are more common than others. These errors are usually due to battery leakage damaging the hardware or flat batteries resulting in random data in the CMOS which requires that a machine is reset to default values. 

Depleted CMOS batteries

Having a depleted CMOS battery usually results in random data residing in the CMOS chip. This can be remedied by replacing the battery or batteries and then resetting the CMOS to its defaults.

  • CMOS RAM checksum invalid (&00109)
  • CMOS RAM checksum invalid (ARM3 fitted) (&00119)

Battery damage related errors

On later machines where they are fitted with a rechargeable battery to power the CMOS, when the battery fails and leaks, it can cause damage to the CMOS chip and/or surrounding area of the PCB.

  • Unreadable CMOS (&10049)
  • Unreadable CMOS (ARM3 fitted) (&10059)
  • RAM Control line failure (&20101) (A300, A400, A500)
  • RAM Control line failure (&20141) (A30x0, A4000, A5000, RiscPC)
An unreadable CMOS is usually the result of either damage to the PCB or the CMOS IC itself. Repairing PCB damage resulting from battery leakage is similar for all Acorn machines as the CMOS chip is of the same type for all.

VIDC Enhancer related errors

VIDC Enhancer related errors are typically non-fatal and the machine will continue to boot successfully after reporting the following errors. These errors usually occur when the VIDC Enhancer has a manual control left in the on position or it is failing to provide a 24Mhz signal to the VIDC chip whilst the POST is being performed.

  • IRQ timing failure (&C009)
  • IRQ timing failure (ARM3 fitted) (&C019)

Resetting the CMOS data to defaults

The Acorn CMOS settings can be reset to a valid set of defaults by holding down the DEL key whilst powering on the computer. That is to say, whilst powered off, hold down the DEL key and turn the power on keeping the DEL key depressed until you see a black screen with a red border indicating that the CMOS has been reset.

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POST error calculator

Acorn RISC computer fault codes are reported via the flashing of the floppy drive light in a sequence of 8 blocks of 4 flashes. Each block of four flashes (e.g. long short short long) represents a set of fault and status bits which can be translated into a list of error codes. 

Select the boxes that represent the long flashes from your computer to derive the fault code and its translation.

Fault code:


RISC OS Rambles

Articles covering a range of topics all about the development of RISC OS from one of its developers Justin Fletcher. Well worth the read!