Fix your Beeb Chip

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Post Reply
User avatar
fordp
Posts: 1036
Joined: Sun Feb 12, 2012 9:08 pm
Location: Kent, England
Contact:

Fix your Beeb Chip

Post by fordp » Wed Nov 25, 2015 12:00 pm

Having seen various broken BEEB threads.

How about using an FM 1608B FRAM with custom firmware in it in the OS ROM Socket as a debug aid?

You would need to put the chip in an extra socket to protect the pins and connect the WE pin separately via a clip.

The firmware would need a working 6502, Address and Data Bus only to start its work testing RAM and exercising other parts of the BEEB.

What do people think to this idea?

FordP
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!

User avatar
leenew
Posts: 4018
Joined: Wed Jul 04, 2012 3:27 pm
Location: Doncaster, Yorkshire
Contact:

Re: Fix your Beeb Chip

Post by leenew » Wed Nov 25, 2015 12:06 pm

Hi,
I don't know about the feasibility of this, but let me be the first to say that I think it sounds like a great, time saving idea for us beeb fixers!
Lee.

User avatar
DutchAcorn
Posts: 2148
Joined: Fri Mar 21, 2014 9:56 am
Location: Maarn, Netherlands
Contact:

Re: Fix your Beeb Chip

Post by DutchAcorn » Wed Nov 25, 2015 1:58 pm

Sounds good, but how is this different from the test rom as described in the service manual (see this thread)?

As Mark pointed out, on a model B, using S25 it is relatively simple to find the defective memory chip without the need of such an aid. However on a B+, a model A or a Master it would be very useful imo.
Paul

Image

User avatar
sweh
Posts: 2110
Joined: Sat Mar 10, 2012 12:05 pm
Location: New York, New York
Contact:

Re: Fix your Beeb Chip

Post by sweh » Wed Nov 25, 2015 2:10 pm

I started to create a very simple ROM. It poked the audio hardware directly and just did "beep" "beep" "beep" "beep".

Worked just fine in BeebEm but failed to work at all on a real Master. So I guess I did something wrong and something needed initialising on real hardware that the emulator didn't need.
Attachments
fake_os.zip
(1.68 KiB) Downloaded 17 times
Rgds
Stephen

User avatar
1024MAK
Posts: 9608
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Fix your Beeb Chip

Post by 1024MAK » Wed Nov 25, 2015 2:20 pm

There is already a test ROM image. A normal EPROM will work, and if you get the right EEPROM, that should work as well.
The existing test ROM code is however, very limited. What is really needed, is for a more comprehensive test program to be written in machine code.

The requirements are:
  • First job of the code is to test that the ROM reads okay, then flash the keyboard LEDs to show that this test passed
  • Next, test if the 6522 used for the User and Printer ports are okay
  • Then test the RS423 port
  • Send a suitable message to both the User port, Printer port and the RS423 port
  • Perform a full RAM test, if it is a pass, display on screen
  • If failures detected, send details to the User port, Printer port and the RS423 port
  • The code up to this point must not require any RAM for variable storage and must not use any instructions that need the stack
  • Once the RAM test has passed, RAM can be used as normal for the following tests
  • Test all the remaking ports and features
Yes, that is a big list. I don't expect the whole lot to be developed anytime soon.

I have been tinkering, but I'm not that good at 6502 code and I keep getting distracted...

The references to the Printer port are so that results can be printed on a dot matrix printer. The RS423 port output can be sent to a PC running a terminal emulator or similar, displaying the results.
The references to the User port can be replaced with the 1MHz port. The idea here is to use a cheap 2 line LCD display to show a limited set of the results.

Mark

User avatar
1024MAK
Posts: 9608
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Fix your Beeb Chip

Post by 1024MAK » Wed Nov 25, 2015 2:26 pm


User avatar
fordp
Posts: 1036
Joined: Sun Feb 12, 2012 9:08 pm
Location: Kent, England
Contact:

Re: Fix your Beeb Chip

Post by fordp » Wed Nov 25, 2015 2:28 pm

1024MAK wrote:There is already a test ROM image. A normal EPROM will work, and if you get the right EEPROM, that should work as well.
The existing test ROM code is however, very limited. What is really needed, is for a more comprehensive test program to be written in machine code.

The requirements are:
  • First job of the code is to test that the ROM reads okay, then flash the keyboard LEDs to show that this test passed
  • Next, test if the 6522 used for the User and Printer ports are okay
  • Then test the RS423 port
  • Send a suitable message to both the User port, Printer port and the RS423 port
  • Perform a full RAM test, if it is a pass, display on screen
  • If failures detected, send details to the User port, Printer port and the RS423 port
  • The code up to this point must not require any RAM for variable storage and must not use any instructions that need the stack
  • Once the RAM test has passed, RAM can be used as normal for the following tests
  • Test all the remaking ports and features
Yes, that is a big list. I don't expect the whole lot to be developed anytime soon.

I have been tinkering, but I'm not that good at 6502 code and I keep getting distracted...

The references to the Printer port are so that results can be printed on a dot matrix printer. The RS423 port output can be sent to a PC running a terminal emulator or similar, displaying the results.
The references to the User port can be replaced with the 1MHz port. The idea here is to use a cheap 2 line LCD display to show a limited set of the results.

Mark
This is just the sort of thing I was thinking.

The benefit of FRAM (or battery backed SRAM) is that you can both read and write the chip you inserted which allows a much more powerful set of routines to run even if the BEEB RAM is dead, which seems to happen a lot there are 16 RAM chips after all.

The easiest output device on the BBC board seems to be the cassette relay!

FordP
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!

JonC
Posts: 731
Joined: Wed May 14, 2014 9:19 pm
Location: Wakefield
Contact:

Re: Fix your Beeb Chip

Post by JonC » Wed Nov 25, 2015 2:30 pm

Umm, wouldn't you want to output the test progress/results to a screen as well as various ports?
Or have I missed something?
Jon
Image

station240
Posts: 864
Joined: Tue Feb 09, 2010 6:11 pm
Location: South Australia
Contact:

Re: Fix your Beeb Chip

Post by station240 » Wed Nov 25, 2015 2:33 pm

What I'd like to see is a beeb version of a port 80 tester (google it). That is something able to deal with almost all the hardware being dead.

Given the most common IC failure is the 6522, followed by ram and 74254 buffer chips, being able to determine if those are faulty would help greatly.

Why not something that plugs over the CPU itself (note the word over), and checks voltages, and emulates the rom decoding to feed the software directly to the CPU.

User avatar
1024MAK
Posts: 9608
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Fix your Beeb Chip

Post by 1024MAK » Wed Nov 25, 2015 2:43 pm

All good ideas, but if a EPROM / EEPROM and a serial lead is all that is needed, the test gear is very cheap.

Dedicated testers exist for some other retro computers, but most people are reluctant to spend too much money on them.

Testers also exist for the Beeb, but they are rare and so expensive.

Mark

User avatar
tricky
Posts: 3902
Joined: Tue Jun 21, 2011 8:25 am
Contact:

Re: Fix your Beeb Chip

Post by tricky » Wed Nov 25, 2015 3:27 pm

I was thinking of adding an LED to the EPROM with a resister and a loop in the EPROM to toggle the led on a specific address line for real emergency output.

Post Reply