Digiac 2000 MAC-III 6502 trainer

[JMohan]

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.

[barnacle]

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

[JMohan]

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.

[GARTHWILSON]

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 .

[BigEd]

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

[BigDumbDinosaur]

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!

[barnacle]

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

[JMohan]

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.

[JMohan]

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.

[BigEd]

That's excellent! I get the impression the serial connection offers some good facilities over and above the keypad options.

[Simatic]

Hi,

I'm quite new to the world of microprocessors.

I have recently acquired a Digiac 2000 with the Z80 board and the MACIII 6502 board. I would like to connect my Digiac MACIII board to a PC using an RS232 to USB converter. I have read all the posts within this topic and there has been some great information.

I have created a cable between the MACIII and the PC and used putty as the console but unfortunately I don't appear to be getting anything.

The cable I have made is connected as follows:-

MAC III PC
--------- ------
(TX1) 1 ----- 2 (RX)
(GND) 3 ----- 5 (GND)
(RX1) 6 ----- 3 (TX)

I have set up the comm port as 9600-N-8-1 Stop Bit but still it doesn't work.

Any help would be greatly welcome.

Kind Regards,

Andrew.

[BigEd]

Welcome! At minimum, I'd suggest you set two stop bits.

But do read the thread: there are at least two important considerations, one is the polarity of the signalling and the other is the voltage level of the signalling.

If you have a scope you can maybe get the board to send something, and capture a trace.

[gfoot]

Simatic wrote:

I have recently acquired a Digiac 2000 with the Z80 board and the MACIII 6502 board. I would like to connect my Digiac MACIII board to a PC using an RS232 to USB converter. I have read all the posts within this topic and there has been some great information.

I have created a cable between the MACIII and the PC and used putty as the console but unfortunately I don't appear to be getting anything.

The cable I have made is connected as follows:-

MAC III PC
--------- ------
(TX1) 1 ----- 2 (RX)
(GND) 3 ----- 5 (GND)
(RX1) 6 ----- 3 (TX)

I assume on the MAC III you're taking the signals directly from the RS232 port. What exactly do you have at the PC end of this? Is it really RS232, or is it just a logic level FTDI USB device?

You can get actual USB-to-RS232 cables, and that may be a better choice for ease of working with this board, given it already has RS232 connectors on it.

[Simatic]

Hi,

Thank you both for your response, I didn't know if this would still generate any posts so thank you.

GFoot - You are correct, I have plugged a 9 pin sub d male connector into the MAC III and connected it to a 9 pin sub d female connector wired up as mentioned below. The female connector is then plugged into a Prolific RS232 to USB converter.

BigEd - I will try the two stop bits and see what happens. Unfortunately I don't have a scope but I know where I can lay my hands on one so I'll let you know how that goes.

I do have an old IBM pc with RS232 ports so I could try it on there.

Kind Regards.