TL:DR; Press <RETURN> after IN#2; you should see IMMEDIATE input on the //c. You CAN use lower-case. You may need to select File > Serial Config > Apple //c ImageWriter Cable option. Try a different USB-DB9 cable if after #Troubleshooting (below) doesn't work.

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AppleWin developer here -- I just went through this exact same issue this week having recently moved and started unboxing my Apple 2 Lab these past few weeks. Sadly I didn't pay attention to the exact ADTPro setup I had before so I was forced to start over from scratch. In the end this actually worked out since I never documented what I did before so here is an opportunity for me to do this now -- and you get the benefit. :-)

Requirements

• MacBook Pro (MBP)
• OSX (I'm running 10.10 but version shouldn't matter)
• Apple //c
• Serial Cable DB-9 (Female) to DIN-5 (male) null modem cable
• USB to RS232 DB9 (male) serial adapter
• ADTPro 2.x

Caveats

NOT all USB-DB9 serial cables work!!! Of the three I have only this one works:

• IOcrest USB 2.0 to RS232 DB9.

These ones do NOT work:

• Sabrent USB 2.0 to Serial (9-Pin) DB-9 RS-232
• Tripp Lite Keyspan High-Speed USB to Serial Adapter, PC & Mac (USA-19HS) -- Note: I believe this one might work -- as I see the serial port with the with provided /Applicatons/Keyspan Serial Assistant. You need to install the kernel driver and reboot OSX (macOS). HOWEVER, every time I plug it in the mouse and OS freeze. You are forced to hold down the power button reboot. :-/

I'm not sure if the serial cable being a null modem is a factor or not.

Solution

1. Make sure the serial cable connected to the port on the floppy drive side. The serial port is on the opposite side of the power supply on the back of the //c. NOTE: You'll see a 2 <phone icon> engraved above the serial port.

2. When you plug in your USB cable and start ADTPro you must see it listed under File > Serial Configuration. For me it was /dev/tty/usbserial.DN000MLG4 but this will vary depending on which brand/model you have. Check your manual for details. Also, I have:

• Speed: 19,200 (Note: You CAN use 115,200)
• [x] Apple //c w/ Imagewriter cable <-- This was THE key for me.

1. Press the Serial button.

2. Select the menu Bootstrapping > ProDOS > Speediboot

3. The docs for ATDPro are missing an essential keypress as it is implied. On the Apple //c side you MUST press RETURN after IN#2:

   IN#2 <RETURN> <Ctrl-A>14B

4. Lower-case DOES work!

5. When you press Ctrl-A the cursor will change to a question mark ?. If you don't see this then the serial card is NOT active.

6. When press type B the cursor will change back to the regular checkerboard glyph. You CAN press <RETURN> after the B as well but it isn't needed like the first one after the IN#2.

7. After pressing OK within ADTPro you should immediately see the CALL -151 and hex input on the Apple //c. If ADTPro sends the file (~30 seconds) but there is NO input on the Apple //c side then your connection is working. See the following Troubleshooting section.

Troubleshooting

• Physical verify the USB-DB9 cable connection is solid. My USB-DB9 adapter has a red LED when hooked up to the MBP -- the other lights turn on when actual sending.
• Serial config: Lower the speed to 19,200 until you verify the higher speed works
• Serial config: Toggling the Imagewriter option
• Serial config: Verify you are using the correct /devdevice
• If you have exhausted all other options try another USB-DB9 cable

Models

I have the following Apple //c models in my Apple Lab. I've tested two of them:

Version	Works
255 (Original)	Yes
0 (3.5 ROM IIc)	Yes
5 (//c+)	? (Haven't tested)

NOTE: You can find which ROM version your //c is via:

? PEEK(64447)

Good luck!

Edit: Replaced //c+ pic with //c one.