Troubleshoot an Alvin BSMT audio board

Pinup
BSMT

Replacement of the PAL

Repair of an Alvin BSMT audio board and replacement of the defective PAL (protected).

Create a substitute GAL by reverse-engeenering.

Presentation

This board ship a Mystery Castle pinball and is located at top right of the pediment.

BSMT

Built on the basis of a CPU 6809 (68B09) and an audio circuit BSMT (derived from the Texas Instruments TMS 320), this card uses a PAL 16L8 (labeled ECP-025) to reduce the number of components. As can be seen in the diagram, this circuit is used only for the address decoding.

Alvin BSMT board PAL decoding diagram

Diagnosis and solution

The board that we own wasn't work at all and the PAL was abnormally heat. Being mounted on support, we have done a cross-test with another functional card.

After exchange of components, it is proves that it was the PAL in question. Unfortunately, the replacement of this programmable chip is problematic because its content is protected and it is not possible to duplicate it.

There is however a solution because looking at the schema, it can be seen that the realized decoding is not so complex. There is 8 output used, but mostly only 8 entries:

  • RW-
  • E
  • A00
  • A11 to A15

What is total 256 possibilities, can therefore be considered a manual scan of the operation (of the functional PAL). This number can also be logically reduced, since it is known for example that the address $FFFF will be necessarily of the ROM.

Achievement

Will replace the PAL 16L8 by a GAL 16V8, a pin to pin compatible component and especially still available.

PAL16L8 GAL16V8 GAL16V8

The 256 patterns analysis is facilitated by the study of the datagram of the 6809. It shows for example that the addresses are valid only when E is "1".

6809 datagrams

It only remains to try all possibilities and establish a truth table, operation facilitated by the use of our test bench. It visualizes the result output by connecting directly with leds on the outputs.

Bench test

Programming the GAL

Once the truth table is known, we must create a JEDEC file for the programming of the GAL. We use WINCUPL (software available from ATMEL) to write the logical equations. This software has a simulator to test the logic of operation expected.

WINCUPL WINSIM

It remains to program the file generated by a programmer (we use the TOP 3100).

Installation

A last quick check on prototype board:
GAL final test

It can then install the new GAL instead of the defective PAL.

Distribution

To know the availability, the terms and conditions of dissemination of our accomplishments, please visit the FAQ (frequently asked questions).

Last update of this page: February 28th, 2015