Lost in a time warp

[clh333]

Question: What SER: port diagnostics are available for the Amiga 500?

Bought an Amiga 500 as a "home computer" in the 1980s; boxed and stored for many years, finally unearthed recently. Although I paid for the equipment I never really played with it much, so I find I am going through with manual in hand testing the hardware and software to see what still works. Many floppies fail w/ bad sector even after cleaning heads of FDs 0: and 1:

Board is revision 5. Unit powers on, POSTs, boots, loads WB 1.3 from df0: or hd0:, a Supra 500 drive and memory expansion through the 80-some pin expansion port (Zorro?) on the left side of the case. Keyboard OK, Video (lo-res) OK, mouse OK (joyport 1 only!), PRT: OK (Panasonic KXP-1180 masquerading as Epson), sound OK.

Serial port not OK. The only term program I have is BaudBandidt, which I have run from the HD and from floppy. As far as I can tell the fault is with the 8520 chip that controls RS232. But I am not sure that I can say so definitively.

With the C64 and 128 machines previously inspected I was able to verify serial communications through a modem, dialing one of the PCs on a second line, with its internal "PC" modem set to answer at ring 1. I established a connection and verified throughput both directions.

Using the same setup the Amiga does not get as far as the dial tone. I have tried three different modems, several cables, straight and null, and even a loopback connector between TXD and RXD. Nothing.

I tried COPY TO SER: just to see if I would get and error message - AmigaDOS informed me of a problem communicating with the printer - but no error arose.

I read the suggestion about swapping the 8520 chips to see if that is the problem, but if I fried one chip with faulty equipment I am afraid I may fry the other as well.

I'm wondering if there is a diagnostic I can apply that is availaqble to me from within the DOS or BASIC. Without serial communications I am unable to transfer any downloads to the Amiga.

Thank you for your reply.
Charles Hudson,
Cleveland, Ohio, USA

[ajk]

@clh333

If you have access to a logic analyzer or an oscilloscope, you could verify if there is any life in the serial port when you try to use BaudBandit.

Otherwise... CIA chips aren't that expensive so you might as well swap them, and get a replacement later on if needed.

[clh333]

Thank you for your reply. This has been a struggle, and if I weren't so stubborn I would have abandoned the project.

Looking back I also realize how far the Amiga progressed beyond that A500. If I am successful here I would like to look at some of the newer machines.

I do have an old scope, one-channel analog Leader, no probes as yet and more importantly, no expertise. But I could get some and give the scope diagnostic a try.

I was thinking more along the lines of addressing the chip to see what is settings are, before and after issuing RS232 commands. Chip pulls on eBay are going for $20-30. Does that sound right?

I'll let you know what I find.

Thanks again,

Charles Hudson
clh333

[ajk]

$20-30 is high, you should almost be able to get a spare A500 motherboard for that. Vesalia (a shop in Germany) sell the chips for €7.90 which is almost the same in dollars at the moment. Here is someone selling the chip for €6 on another forum. Amibay is a great source for anything Amiga related and you can avoid inflated eBay prices most of the time.

[clh333]

Originally Posted by ajk

$20-30 is high, you should almost be able to get a spare A500 motherboard for that. Vesalia (a shop in Germany) sell the chips for €7.90 which is almost the same in dollars at the moment. Here is someone selling the chip for €6 on another forum. Amibay is a great source for anything Amiga related and you can avoid inflated eBay prices most of the time.

Thank you for the Vesalia reference. I was unaware. I purchased two chips from a DE eBay auction and am awaiting delivery; paid about $22 USD with delivery. But who knows where these chips have been? If they turn out to be faulty, I will refer to Vesalia for replacements.

Once again, thank you for your suggestions.
Charles Hudson
clh333

[clh333]

I obtained two 8520 chips and replaced U8 on the motherboard. I then attempted once again to use the serial port to communicate with a Hayes 9600 modem, without success.

I know the Hayes is operational, I know the telephone line has a signal, and I have tried cabling with and without a null-modem adapter. I assume the 8520s are good since the keyboard, printer etc. are all functioning. So I'm wondering if it's possible that U38 and / or U39 are at fault. I believe these are NAND and NOT gate ICs.

Or do I need a special cable? In the Introduction to the Amiga Technical Reference Section A-4, "Serial Connector", a warning is given "Pins 9 and 10 ... are used for external power. Connect these pins ONLY if power from them is required by the external device." Would use of a 25-wire cable be the source of the problem?

Thank you for your assistance.
Charles Hudson
clh333

[clh333]

Mis-spoke: U38 and U39 are Quad line driver and receiver 1488 and 1489.
-CH-

[Jope]

The line drivers are a common fault. As long as the voltage pins are not connected on your cable, you're fine.

[clh333]

Originally Posted by Jope

The line drivers are a common fault. As long as the voltage pins are not connected on your cable, you're fine.

Thank you for your suggestion. I'll try replacing the drivers.

Texas Instruments notes that the devices are MOS, have limited static protection and should be stored in conductive foam. Given the port's limited top rate of 9600 baud, is there a more robust, i.e. non-MOS IC that could be substituted?

I have two Amiga 500s, both boards Rev 5, and both exhibit the same problem. I thought it highly unlikely that both could have the same problem with their ICs but I guess that's the place to look next. I could easily have zapped them with static electricity without realizing my error. It has been very cold (-30C) and dry here this winter.

Thanks again,
Charles Hudson
clh333

[Jope]

The A500 can do 19200 sustained over the serial port.. The line drivers are actually not the first things to give out, it's more of a DMA stealing CPU cycles problem than slow line drivers problem.

You could probably replace them with a MAX232, but I've not looked at the datasheet/A500 schematics that closely with this in mind.

[clh333]

I think the MAX232 is a combined driver and receiver; I don't know if it is pin-compatible. I also read that the "slew rate" for these devices is controlled by a capacitor, which Commodore has undoubtedly wired into the circuit. Probably best to replace like with like. Jameco has the 1489 but not the 1488. Guess I'll have to roll the dice with eBay for that.

[Jope]

No, it is definitely not pin compatible, you would have to rewire that part to match the max232..

[clh333]

Found them on DigiKey.

[clh333]

Originally Posted by clh333

Found them on DigiKey.

Follow up to previous posts:

Thanks to all who contributed their suggestions and advice. I obtained the 8520 chips from an eBay sale in Germany; delivery took about two weeks and, with trepidation, I tried installing one in one of the Amigas, both of which were unable to communicate with my Hayes 9600 modem.

The chip swap was unsuccessful, alas, but I studied the circuit diagram in the Amiga documentation and concluded that the other possible culprit might be the line driver ICs. I ordered (several) replacements and some 14-pin sockets as well.

In case anyone else is tempted to try this, let me offer my advice, hard-won: Without sophisticated repair equipment it will be nearly impossible to de-solder the old chips, all 14 pins at a time. What I did, after some early mis-steps, was to cut the pins using a very small pair of diagonal cutters and remove the chip body.

After that it was relatively simple to heat one pad at a time from the under side of the board and remove the pin from the top side with needle-nosed pliers. I used a solder-bulb and a dental pick to clean the holes before fitting the sockets.

I put the Amiga board back in the case but unknowingly I mis-aligned the floppy-drive cable on the board header. Needless to say the A500 would not boot. It was necessary to disassemble the computer along side its twin to discover the mistake.

But it booted up, eventually, and (after powering off again) I connected the modem, loaded the telecom software and made a test call. Q.E.D., three months in the making.

I was messing around with the Amigas' serial ports in the dead of winter when static electricity could be generated by breathing. I also ignored the Amiga documentation's warning to always power down before connecting or disconnecting anything to the Amiga. To this I would add that the Amiga's serial port is non-standard, vis-a-vis RS-232, supplying voltage on pin 9, so I would be very careful about the cable I used.

Picture attached.

Thanks again,
Charles Hudson
clh333

[ajk]

Great to hear that you got it working

[jbenam]

Excellent read, Charles. Definitely one of the best written threads here on EAB.

Glad you got it solved. Kudos to Jope and ajk for their help in reaching the solution of this enigma