Early Apple IIc with serial port timing issues

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Early Apple IIc with serial port timing issues

Hi all, I have an Apple IIc that seems to have the serial port timing issues described here:

https://en.wikipedia.org/wiki/Apple_IIc#Serial_port_timing_fix

I found this post here on apple fritter (many years old):

https://www.applefritter.com/content/c-74ls161-ttl-oscillator-fix

Looking at my own board I can see that it does not have the 1.8432MHz crystal installed it also does not have any indications on the board for an optional installation of one.

I am experiencing the serial port timing issues, for me ADTPro works just fine but when trying to use a terminal emulator (any terminal emulator) and regardless of what it's connected to (rs232 wifi adapter, raspberry pi running tcpser, a PC running tcpser, or even just another PC with a terminal emulator running on that) I can only get it to work at 300 baud (or below)

I have tested my serial cable to see if there's any bad connections and it all seems fine so given all of this it seems like the serial port timing flaw is the culprit.

I went ahead and did this mod by cutting the trace going into pin 6 of the "ser2" (6551) chip and soldering the crystal directly onto the legs of the 6551 (pins 6 and 7). using a little masking tape to make sure the crystal doesn't short out anything on the board.

Well the good news is I didn't break anything the bad news is nothing has changed behavior wise.  I still can't get a serial connection above 300 baud.  ADTPro (using same serial connection) still works fine.  I'm having a hard time finding anyone who has tried to do this mod, it seems as long as people can run ADTPro and get Oregon Trail to work they're happy, well I want to do BBSing on this thing so for me I want to pursue it further.

Has anyone actually attempted this fix?  Any advice?

 

 

 

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Interesting issue.  ADTPro

Interesting issue.  ADTPro manipulates the serial port hardware directly and transfers happen at 115,200 BPS.

So I'm not sure your issue is a hardware problem.

 

What terminal program software are you attempting to use?  With an Apple IIc, IIe, or IIGS I would not bother with anything other than ProTerm 3.1 unless there's a compelling reason to use something else.

 

PT3.1 does an awful lot.

 

Also, what sort of WiFi modem are you using?

 

 

 

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Mostly ProTerm 3.1 but I have

Mostly ProTerm 3.1 but I have experiemented with other term programs as well.  There's differences in what max baud rate each program supports but either way above 300 baud I get the same result.  I've also tried experimenting with different port settings like 7 data bits, 1 or 2 stop bits and parity. Unfortunately TcpSer doesn't seem to have options like that so it's stuck with 8N1, but when experimenting connecting to a PC directly I can try those settings but doesn't seem to change the behavior.

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I have a wimodem232 and the

I have a wimodem232 and the early version of the //c with the "bad" serial port. I can confirm I'm able to get at least 2400 baud working with proterm 3.1.  I don't recall if I tried 4800 or not. 9600 was a no go for sure. I recently picked up a rom4 //c which obviously has the "good" serial port, but haven't tried the wimodem with it yet. 

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hmm okay I had rom 255 but

hmm okay I had rom 255 but recently replaced it with rom 0, I guess I can try putting version 4 in there and see if that has any effect on this issue. I didn't read anything about the differences in rom versions that affected the serial port but it's worth a shot. 

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I was only referring to the

I was only referring to the rom version as a way of indicating the hardware was newer on my other //c. I wasn't clear in that point. I highly doubt the rom version is your issue. I was just pointing out the 255 with the "bad" serial timing is capable of at least 2400 with a wifi modem. 

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Oh I see.  I guess I'm back

Oh I see.  I guess I'm back to having no idea what's going on then. 

 

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what to do next

Rather than bump your head against the wall, I suggest you pickup an //c without that problem: It is unfortunate you got an old one with the wrong crystal.

I now have 3 functioning //c's from picking up ones "for parts only" on eBay some years back and one of them even had the Applied Engineering Z80 board installed.  I already have the Z-RAM.

The problem was picked up by Apple early and they swapped out motherboards for free if you purchased a modem at that time.

I was an early //c adopter, but my motherboard was either fixed or swapped out for free because I purchased the 3.5 drive for the //c and never looked back!

I did buy a WiFi Modem a year back, but other than connecting to BBS' in emulation, I am not sure what else to do with it. It works on my Mac SE/30 as well.

My experiences with ADTPro were many years ago when I moved all of my data disks/files over to my Mac to do work on them with Virtual ][.

I had my //c connected to a 2007 MacMini with a USB to serial cable and it worked like a charm.

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Note that crystals are not

Note that crystals are not all made the same. The crystal must have the correct frequency, but it must also have the correct type of resonance (series or parallel) and load capacitance. Otherwise it will not oscillate in the circuit as intended. Powered (4-pin) oscillator units are more forgiving because they are self-driven and not as dependent on circuit characteristics. The schematic doesn't specify these parameters; perhaps they are detailed on a bill of materials.

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Hmm yeah I have no idea how

Hmm yeah I have no idea how to find out the correct type of crystal, all I know is the frequency. 

Maybe someone who has a board with a factory installed crystal can determine.

 

I'd really like to get this computer working rather than just buying another one.  I just don't like to do that especially with vintage hardware that isn't produced anymore.

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