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JMohan
PostPosted: Wed Feb 07, 2024 8:27 am 
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Hi Neil,
I connected the logic analyser to serial port CH.A coming out of the Digiac board. as I mentioned in a previous post the pinout is non standard pin 1 is TX1 Pin 6 is RX1. I had to make a custom cable which I broke out onto a breadboard so I could connect the ftdi ft232rl adapter and the logic analyser. The chip on the digiac board that handles the serial communication is a motorola mc2681 duart. The output from the duart goes through a max232 ic and int o the serial port.
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barnacle
PostPosted: Wed Feb 07, 2024 9:09 am 
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Yup, I suspect you're looking at an inverted RS 232 signal if you're looking at the signal on your breakout box, after the D-9 connector.

But if your FTDI adaptor is an RS232 adaptor and not a UART adaptor, it should correct that apparent inversion (UART can be expected to idle at +5v (or +3v3 for 3v3 systems) while the RS232 signal idles at -3 to -15v)

Neil
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JMohan
PostPosted: Wed Feb 07, 2024 7:38 pm 
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I measured the voltage at idle at the db9 connector on the digiac it reads -9v

I did some research, apparently you can reprogram the firmware on the ftdi ft232r to invert the tx and rx lines.

after several hours of messing around I reprogrammed my converter with inverted signals.
Now when I try to connect to the digiac via serial it does nothing, the digiac board wont go into terminal mode.
I tested the ftdi adaper by connecting tx and rx together and it works.
I'm running out of ideas.
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GARTHWILSON
PostPosted: Wed Feb 07, 2024 8:20 pm 
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Joseph, it's not clear to me if you understand the difference between just a UART and RS-232 (most accurately "TIA-232," but everyone understands "RS-232").  It's not just about inverting versus not inverting.  RS-232 levels are much higher voltages than just normal logic levels, and they're positive and negative too.  Pretty standard is to go plus and minus twelve volts (although there's a wide acceptable range), to become more immune to noise getting into the line; and connecting an RS-232 output to a standard logic input will probably damage the latter.  I certainly understand the common desire to forgo the RS-232 line drivers and receivers; but unfortunately this has led to confusion and incompatibility, and sometimes damage.  Your FTDI adapter needs to be one that that has the RS-232 line drivers and receivers.  I have a somewhat 65xx-oriented RS-232 primer at http://wilsonminesco.com/RS-232/RS-232primer.html .

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BigEd
PostPosted: Wed Feb 07, 2024 10:27 pm 
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I'm pretty sure you can get a cheap and cheerful level converter for this situation. (Which also inverts.)

I think I might have one of these - I imagine there are cheaper versions
https://www.sparkfun.com/products/449

Or you can roll your own
http://picprojects.org.uk/projects/simpleSIO/ssio.htm
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BigDumbDinosaur
PostPosted: Thu Feb 08, 2024 3:41 am 
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Mohan wrote:
I measured the voltage at idle at the db9 connector on the digiac it reads -9v...

It appears the serial interface on the Digiac unit is TIA-232.

TIA-232 operates at a nominal plus and minus 12 volts DC with respect to ground, with generally-accepted operating conditions supporting a voltage range of 3 to 25 volts.  When the voltage is positive with respect to ground, the TIA-232 circuit is in the “space” condition, which may be thought of as a logic 0.  When the voltage is negative with respect to ground, the circuit is in the “mark” condition, which is considered a logic 1.

On the UART side, a space is zero volts, which is the “opposite” of what is on the TIA-232 side (+3 to +25), but still representing a logic 0.  A mark is a high, generally close to VCC, again the opposite of what is on the TIA-232.

Due to the differences between voltage and polarity on both sides, line drivers, such as the Maxim MAX232 or the older 1488/1489 devices, are required to couple the UART to the TIA-232 link.  The drivers act as level converters and logic inverters at the same time.  As a fairly general rule, you cannot safely connect a TIA-232 circuit directly to a UART—this is aside from the inverted logic meanings.  UARTs operate at 5 or 3.3 volts, always positive with respect to ground.  A TIA-232 circuit operating at full nominal voltage will drive the UART’s inputs well beyond their ratings and likely cause damage.

BigEd wrote:
I'm pretty sure you can get a cheap and cheerful level converter for this situation. (Which also inverts.)

I think I might have one of these - I imagine there are cheaper versions
https://www.sparkfun.com/products/449

Or you can roll your own
http://picprojects.org.uk/projects/simpleSIO/ssio.htm

Those are okay if the only connections you want to make are RxD and TxD.  They have no provisions for CTS/RTS flow-control.  In most cases, attempting to operate above 9600 bpS without hardware flow control will lead to errors during sustained transfers.  As always, caveat emptor!

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barnacle
PostPosted: Thu Feb 08, 2024 6:26 am 
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I was unable to find either a circuit diagram or a sufficiently clear photo of the Digiac circuit board, but from the photos I did find it looks as if there are two UART-RS232 converter chips between a 40-pin part and the sockets. You could hook a UART signal at their inputs, if this is so.

Neil
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JMohan
PostPosted: Thu Feb 08, 2024 1:43 pm 
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I'm just going to wait until the serial card that I ordered for my pc to arrive, I'll make a custom cable for it and report back my findings.
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JMohan
PostPosted: Sat Feb 10, 2024 1:08 pm 
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Attachment:
Digiac_MACIII_6502-first-contact.png
Digiac_MACIII_6502-first-contact.png [ 84.46 KiB | Viewed 1023 times ]


Good news, everyone it is working, I can now communicate with the board via serial terminal.

Thank you all for your advice, suggestions and constructive criticism.
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BigEd
PostPosted: Sat Feb 10, 2024 1:45 pm 
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That's excellent! I get the impression the serial connection offers some good facilities over and above the keypad options.
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