The XUM1541 allows to connect devices that use the Commodore serial bus (IEC bus) to modern computers via USB. You usually use it to create images of old 1541 or 1571 floppy disks by using a software called OpenCBM.
The subdirectory "gerber-files" contains a Zip file you can use to order the PCB from PCB manufacturers like PCBWay or JLCPCB. Simply upload the Zip on their web page. When you plan to use the parallel port option, you have to order a PCB that has the default thickness of 1.6mm, and I recommend to order a surface finish of ENIG with gold fingers and chamfered edges, because it is much more durable than the default HASL finish.
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1x Pro Micro ATmega32U4 5V 16MHz Board.
There are two variants of this board, a 5V variant with 16MHz clock frequency, and a 3V variant with 8MHz. Make sure that you get the 5V variant! Also make sure that you get a Pro Micro Board based on the ATmega32U4. There are Pro Micro Boards available based on the Raspberry Pi Pico, these do not work!
These boards come in at least three different widths. This XUM1541 PCB is designed to support all three widths (the distance between the two rows of pins can be either 15.24mm, 17.78mm or 20.32mm).
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2x 1x12 Pin Headers Through Hole Straight 2.54mm pitch
These go onto the Pro Micro board. They should be included with the Pro Micro board.
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(Optional) 2x 2x12 Pin Sockets Through Hole Straight 2.54mm pitch.
These are used to connect the Pro Micro Board to the XUM1541 PCB. They are optional, you can solder the Pro Micro directly to the PCB instead.
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1x 6 Pin DIN socket Right Angle PCB mount.
The footprint is for Lumberg 010599 06 or equivalent sockets.
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(Optional) 1x 74LS06 Hex Inverter Buffers DIL-14 (U1).
Using this buffer chip is strongly recommended, especially when your're daisychaining more than one device on the IEC bus, but it is optional. When you install it, you should also install the capacitor C1 and the register network RN2. When you do not install it, you have to close the corresponding 5 solder jumpers on the bottom of the XUM1541 PCB.
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(Optional) 1x IC Socket DIL-14 (U1).
Socket for the 74LS06. It is optional, you can alternatively solder the IC directly onto the XUM1541 PCB.
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(Optional) 1x Ceramic Capacitor 100nF 2.5mm pitch (C1).
This is the blocking capacitor for U1. It is optional, you only need to install it when you install the 74LS06.
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(Optional) 1x Resistor Network 5x100kOhm SIL-6 Star Connection (RN2).
This resistor network is only needed when installing the 74LS06. It pulls the inputs of the 74LS06 to ground.
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(Optional) 1x Resistor Network 5x4.7kOhm SIL-10 Single Resistors.
This resistor network is not used when you do not install the 74LS06.
When you do install the 74LS06, you may optionally install this resistor network to limit the amount of current on the IEC bus. When you install the 74LS06 and decide to leave this resistor network unpopulated, you have to close the corresponding 5 solder jumpers on the bottom of the XUM1541 PCB.
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(Optional) 1x Push Button 6x6mm (SW1).
A reset switch that resets the Pro Micro, i.e. puts it into bootloader mode for 8 seconds. Completely optional, but having this switch makes installing a new firmware much more simple.
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(Optional) 2x LEDs 3mm or 5mm (D1 and D2).
These are the power and activity LEDs. You can use any color you want, e.g. red for power and green for activity. These LEDs are optional, there are already corresponding LEDs on the Pro Micro board. When you install them, you also have to install matching series resistors R1 and R2. The activity LED requires an installed 74LS06!
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(Optional) 2x Resistors size 0207 (R1 and R2).
These are the series resistors for the LEDs D1 and D2. Of course they are only required when you actually install these LEDs! Their value depends on the types of LEDs you use (color) as well as the brightness you want to get. I recommend that you use a breadboard and check which values are ok for you.
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(Optional) 1x 1x3 Pin Headers Through Hole Straight 2.54mm pitch
This is a header for a serial TTL connection to the ATmega. This is only needed when you want to debug the XUM1541 firmware running on the microcontroller. You will probably not need this header. When you use it, keep in mind that it uses two pins also used by the parallel port of the XUM1541, so you will not be able to use both simultaneously.
The first step is to solder the two 1x12 pin headers to the Pro Micro board. I recommend to use a breadboard to make sure that the pin headers are correctly aligned. But be careful not to drop solder onto the breadboard!
There are three options for building the XUM1541:
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The simple version without U1, C1, RN2 and RN1. I generally do not recommend this option. You should only connect one single device to the IEC bus when using this option. You have to close the five solder jumpers below the U1 footprint on the bottom of the PCB, and that is all.
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The version with U1, C1 and RN2, but without RN1. The 74LS06 on U1 is a buffer chip, i.e. it is used to amplify the signals of the Pro Micro and to protect the ports of the Pro Micro. It is strongly recommended to install U1 when you plan to connect more than one device to the IEC bus. RN2 are pull-down resistors to force the inputs of U1 to GND level when not driven by the Pro Micro.
You can either install a DIL-14 socket for U1, or you can directly solder the 74LS06 IC to the XUM1541 PCB. I personally prefer to use sockets. Make sure that the little notch of the socket points to the direction marked on the silk screen (to the right on the photo below).
I recommend to wait until you are finished with soldering before you install the 74LS06 into the socket. Again make sure that the notch of the IC points to the direction of the notch of the socket (to the right on the photo).
Now you have to install the blocking capacitor C1 next to U1.
And you have to install the resistor network RN2 next to U1. Be careful, the orientation of this resistor network matters! There is a small dot on the resistor network that marks pin 1. Pin 1 is also marked on the silk screen of the PCB (left on the picture below).
Because you are not installing resistor network RN1, you have to close five solder jumpers on the bottom of the PCB.
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The full version with U1, C1, RN2 and RN1. This is the recommended version. RN1 limits the current between the IEC bus and the Pro Micro and protects the ports of the Pro Micro as well as your valuable retro devices connected to the IEC bus. The electric circuit is exactly the same as used for the ZoomFloppy available for purchase from Retro Innovations.
Additionally to installing U1, C1 and RN2 (see documentation for variant 2 above), you have to install resistor network RN1. Unlike resistor network RN2, the orientation of the network does not matter, but it is nevertheless good practice to solder pin 1 with the dot into the hole marked on the silk screen (at the top in the picture below).
This step is completely optional. You can solder the Pro Micro board directly onto the XUM1541 PCB, but I prefer to install Pin Sockets.
There are two rows of through-holes on the PCB for each side of the Pro Micro board. Each pair of rows has the same electrical connections, so you can freely choose which one to use to adjust to different widths of the Pro Micro. When the distance of the two rows of pins of your Pro Micro is 15.24mm, use the two inner rows. When the distance is 20.32mm, use the two outer rows. When the distance is 17.78mm, you use the outer row on one side, and the inner row on the other side.
To solder the pin sockets straight, I recommend to stick them onto the pin headers of the Pro Micro and put them onto the PCB. But again, be careful not to drop solder onto the Pro Micro!
Now solder the 6 pin DIN socket onto the XUM1541 PCB. This is a straightforward task.
The 1x3 pin header for debugging is optional. It provides a serial interface with TTL levels and might be helpful when you are making changes to the firmware of the Pro Micro, because you can use a serial terminal on another computer to receive debug output from the Pro Micro.
The LEDs for activity and power as well as their respective series resistors are also optional. There are already corresponding LEDs on the Pro Micro board. I intentionally do not give any values for the resistors here, because they depend on the type of LED (color) you use as well as the level of brightness you want to get. I recommend that you test different resistor values on a breadboard to determine the right resistor values.
Important: The activity LED requires U1 to be installed!
When soldering the LEDs, keep in mind that the orientation of the LED matters. There is a side of the LEDs that is flattened. This side is also marked on the silk screen (the side facing away from the resistors). Unfortunately, this might be little bit hard to see. Another hint is the length of the LED leads - the long lead is the anode (+) and goes to the resistors, the short lead is the cathode (-) and must point away from the resistors.
The reset button SW1 is optional. Pressing the reset button switches the Pro Micro into boot loader mode for 8 seconds. Having a dedicated reset switch makes installing a new firmware on the ATmega32U4 much simpler.
After soldering the XUM1541, you have to install the firmware onto the Pro Micro. You will find the precompiled firmware in the OpenCBM GitHub repository. Make sure that you fetch the correct firmware:
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When you did not install U1 (74LS06), you have to install the firmware "xum1541-PROMICRO-v??.hex", where "??" is the version number.
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When you did install U1, you have to install the firmware "xum1541-PROMICRO_7406-v??.hex".
There are many different ways to install the firmware. You can use an Atmel ICSP programmer (or another Arduino), or you can install the firmware via the USB port. The Pro Micro usually comes with the Arduino Leonard boot loader pre-installed. You switch the Pro Micro into boot loader mode by pressing the reset button. When you did not install the reset button, you have to short the RST pin of the Pro Micro board to GND. The Pro Micro stays in boot loader mode for only 8 seconds, so you have to be quick now! I used a software called "avrdude" to install the firmware. avrdude is also used by the Arduino IDE, so a simple way to get avrdude is to install the Arduino IDE. I used the following command line to install the firmware:
avrdude -v -patmega32u4 -cavr109 "-P/dev/tty.usbmodem2101" -b57600 -D "-Uflash:w:xum1541-PROMICRO_7406-v08.hex:i"
Of course you have to adjust the command line, especially the serial port!
For installing OpenCBM, I recommend to have a look into the official "OpenCBM Users Guide" for more information.
XUM1541 Pro Micro (c) by Patrick Dähne
XUM1541 Pro Micro is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
You should have received a copy of the license along with this work. If not, see https://creativecommons.org/licenses/by-nc-sa/4.0/.