ModbusRTU Test Shield

A shield for Arduino boards that provides an RS-485 transceiver, RS-232 transceiver, and a selection of buttons, lights, and knobs. This board was designed to aid in testing the ModbusRTUSlave and ModbusRTUMaster libraries.

• This shield is designed to work with both 5V and 3.3V Arduino boards.
• A 3-position removable screw terminal is used as the RS-485 interface.
• A RJ12 connector is used as the RS-232 inteface. The pinout of this connector is designed to make it compatible with Automation Direct products.
• The potentiomter knobs can be easily removed so you can stack other shields on top of this one.
• There is a switch to easily change between using a HardwareSerial port or a SoftwareSerial port.
• The selected serial port is connected to both the RS-485 and RS-232 transceivers, so not confugration changes are needed to switch between them.

Note

This design is distributed WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY, SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE.

This board is designed to use only one serial interface (RS-485 or RS-232) at a time. Attempting to use both at the same time is unlikely to damage the board, but could lead to unexpected behavior.

Pinouts

Arduino Pin Usage

Pin	Function	pinMode
0	HardwareSerial RX	INPUT (Set by HardwareSerial)
1	HardwareSerial TX	OUTPUT (Set by HardwareSerial)
2	Button1 (BTN1)	INPUT_PULLUP
3	Button2 (BTN2)	INPUT_PULLUP
5	LED1 (L1)	OUTPUT
6	LED1 (L2)	OUTPUT
7	LED1 (L3)	OUTPUT
8	LED1 (L4)	OUTPUT
10	SoftwareSerial RX	INPUT (Set by SoftwareSerial)
11	SoftwareSerial TX	OUTPUT (Set by SoftwareSerial)
13	Driver Enable (DE)	OUTPUT (set by ModbusRTUSlave or ModbusRTUMaster)
A0	Potentiometer1 (POT1)	INPUT
A1	Potentiometer2 (POT2)	INPUT

Unused Pins: 4, 9, 12, A2, A3, A4, A5

RS-485 Interface (J5)

Pin	Function	Electrical Type
1	RS-485 D+	Bidirectional
2	RS-485 D-	Bidirectional
3	GND	Passive

RS-232 Interface (J6)

Pin	Function	Electrical Type
1	GND	Passive
2	5V	Power Output
3	RS-232 RX	Input
4	RS-232 TX	Output
5	VBUS	Power Input
6	GND	Passive

Documentation

• Schematic
• KiCAD Source Files
• STEP 3D Model

Components

Circuit Board

Specifications

• Board Length: 68.58mm
• Board Width: 53.34mm
• Board Thickness: 1.6mm
• Substrate Material: FR4
• Copper Layers: 2
• Copper Thickness: 1oz
• Soldermask Color: Any (Blue Suggested)
• Smallest Hole Diameter: 0.6mm
• Smallest Trace Width: 0.2mm (7.8mil)
• Smallest Trace Spacing: 0.2mm (7.8mil)

Suppliers

Below are some circuit board manufacturers I have used in the past. Generally you will need to upload the upload the gerber and drill files to their websites in a .zip folder.

• OSH Park is a US based circuit board fabrication service. They tend to be more expensive than other manufacturers and the number of options they give you are limited (If you use this manufacturer, I hope you like purple).

• Seeed Studio Fusion makes circuit boards in China. They have a lot of options when ordering; please refer to the specifications above when making your choices. They do have a circuit board assembly service as well, but I am not especially familiar with it.

Purchase Components

Reference	Qty	Manufacturer	Part Number	Description	Notes
C1-C6	6	KEMET	ESS105M050AB2EA	1uF Aluminum Electrolytic Capacitor
D1-D4, D6-D8	7	Würth Elektronik	151031YS06000	Yellow LED
D5	1	Würth Elektronik	151031VS04000	Green LED
D9, D10	2	Vishay	BAT43-TAP	Schottky Diode
D11	1	Vishay	SB260S-E3/54	Schottky Diode
J1-J4	1	SparkFun Electronics	PRT-11417	Stacking Socket Header Kit	This kit contains all the connectors for the Arduino interface.
J5	1	Phoenix Contact	5434557	3-Position Terminal Block Plug
J6	1	Amphenol ICC (FCI)	54601-906WPLF	RJ12 Jack
R1-R8	8	Stackpole Electronics	CFM14JT1K00	1K Ohm Resistor	These are generic 1K Ohm resistors; smilarly sized components of the same value should work.
R9	1	Stackpole Electronics	CFM14JT10K0	10K Ohm Resistor	This is a generic 10K Ohm resistor; a smilarly sized component of the same value should work.
RV1, RV2	2	Piher/Amphenol	PT10MV10-103A2020-E-S	10K Ohm Potentiometer
RV1, RV2	2	Piher/Amphenol	JPEPL6052INI	Potentiometer Knob
SW1-SW3	3	Omron	B3F-1020	6mm Pushbutton Switch	This is a generic 6mm pushbutton switch. Feel free to substitute.
SW4	1	C&K	OS202011MS2QN1	DPDT Slide Switch
U1	1	Analog Devices	LT1785ACN8#PBF	RS-485 Half-Duplex Transceiver	This is a fairly expensive RS-485 transceiver, but it is what I had on hand. Something like a Renesas ISL81487EIPZ should work just fine.
U2	1	Texas Instruments	MAX232N	RS-232 Transceiver
XJ5	1	Phoenix Contact	5430438	3-Position Terminal Block Header
XU1	1	Assmann WSW Components	AR 08-HZL/01-TT	8-Position DIP Socket
XU2	1	Assmann WSW Components	AR 16-HZL/01-TT	16-Position DIP Socket

You may also want termination resistors for RS-485; 120 Ohms is a common value.

Suppliers

Below are some electronics suppliers I have used in the past:

• Digi-Key is my usual go-to supplier for electronic components. Their website makes it pretty easy to sort through parts.

• Mouser is very similar to Digi-Key in a lot of ways. They sometimes have better pricing, and they have excellent customer support.

• Newark tends to have a bit different selection than Digi-Key and Mouser, especially in the area of connectors.

• Octopart is not a supplier per se; it is essenitally an electronic component search engine. This can be a very helpful tool when looking for parts, especially somewhat unusual ones, like extended socket headers.

All of these suppliers have a feature on their respective websites where you can upload a spreadsheet file, such as ModbusRTU-Test-Shield-BOM.csv, to populate the cart or find parts.

Assembly

Tools and Supplies

Item	Optional	Notes
Soldering Iron	No	Make sure you use a good quality soldering iron. A poor quality soldering iron will give you a miserable soldering experience.
Flush Cutters	No	These are used to trim component leads.
Flux Core Solder	No	Leaded solder is easier to work with, but lead-free solder is better for the environment.
Solder Flux	Yes	Extra flux will make the solder flow better. I don't usually use it when through-hole soldering, but it can be helpful.
Solder Wick	Yes	This is used to remove solder from places you don't want it.
Solder Sucker	Yes	If you need to de-solder a component, this, along with solder wick, is the tool you want. This can quickly remove most of the solder from a joint.
Arduino Shield	Yes	This can be used to hold the stacking socket headers in place while being assembled.
Isopropyl Alcohol	Maybe	This is used to remove flux. It is not needed if you are using no-clean flux.
Dish Soap	Maybe	This is used to remove flux residue. It is not needed if you are using no-clean flux.

Note on Flux
Traditional flux if left on the assembled circuit board could cause some serious issues. It can potentially cause shorts or significant parasitic capacitance. It is also somewhat corrosive and can, over time, damage your circuit board. This sort of flux must be cleaned off the board after assembly.

To address these issues there are no-clean fluxes, that can be left on the circuit board assembly and are unlikly to cause these issues. There are also water-soluble fluxes that can be cleaned off with hot water, though I have found that some isopropyl alcohol can speed up the process. There are also fluxes that are both no-clean and water soluble; this is what I like to use.

Assembly Steps

Except for the socketed parts, all the components are placed on the front side of the board and solderd on the back side of the board. These steps go through soldering each component, starting with the shortest.

The interactive HTML BOM may be helpful to you in this process. It graphically shows where each component goes on the board. You will need to download it in order to view it; GitHub does not render HTML files in repositories.

1. R1-R9 (Resistors) and D9-D11 (Diodes)

Note

There are two resistor values used.

The 8 resistors furthest from the diode footprints (R1-R8) are 1K Ohm resistors.
The color code on these will be orange, orange, brown, gold.

The resistor right next to the diode footprints (R9) is a 10K Ohm resistor.
The color code on this resistor will be brown, black, orange, gold.

Two different diode parts are used on this board.

The smaller signal diodes (D9, D10) are placed next to the resistors.

The larger power diode (D11) is placed right next to the RJ12 footprint.

• Bend the compnent leads so that the parts will fit in their footprints.
• Place the components.
• Bend the leads on the backside of the board to hold them in place.
• On the backside of the board, solder the leads to the pads.
• Trim the leads using the flush cutters.

2. XU1-XU2 (Integrated Circuit Sockets)

You will likely need to install the sockets one at a time.

• Place the socket so that the notch matches up with the notch shown on the circuit board.
• Flip the board over and solder two pins in opposite corners of the socket.
• Check that the socket is straight and seated correctly on the board. If everything looks good proceed, otherwise, reheat your solder joints one at a time and adjust the socket position.
• Solder the remaining pins.

3. SW1-SW3 (Push-button Switches)

• Place the switches; these have kinked leads, so they should stay once placed.
• Turn the board over and solder the leads to the board.

4. D1-D8 (LEDs)

Note

The ON LED (D5), is green.

All the other LEDs (D1-D4, D6-D8) are yellow.

• Place the LEDs in their respective footprints with the short lead (cathode) going through the squarish pad.
• Bend the leads on the backside of the board to hold them in place.
• Solder the leads.
• Trim the leads.

5. RV1-RV2 (Potentiometers)

You will likely need to install the potentiometers one at a time.

• Place the potentiometer.
• Flip the board over and solder the pins.

6. XJ5 (Terminal Block Header)

• Place the terminal block header.
• Solder one of the pins.
• Check the placement of the header; reheat the solder joint and make adjustments if necessary.
• Solder the remaining pins.

7. C1-C6 (Capacitors)

• Place the capacitors, ensuring the - symbol on the capacitors is facing away from the + symbol on the board. For capacitors, the positive lead goes through the squarish pad and the negative lead goes through the circular pad.
• Bend the leads on the backside of the board to hold the capacitors in place.
• Solder the leads.
• Trim the leads.

8. J1-J4 (Arduino UNO Interface)

• If you have a shield for the Arduino UNO, place the stacking socket headers on its pins, then place the socket header pins through the holes in the circuit board. This will make it easier and faster to install the sockets. If you don't have a shield you will likely need to install the sockets one at a time.
• Carefully flip the board over the board, holding the socket headers in place.
• Tack solder the end pins of each socket header.
• Check the placement of the headers and make adjustments as needed.
• Solder the remaining pins.
• If you used a shield, remove it now.

9. SW4 (Slide Switch)

• Place the slide switch.
• Flip the board over and solder two of the smaller pins in opposite corners.
• Check the placement of the switch, making adjustments as needed.
• Solder the remaining pins.

10. J6 (RJ12 Jack)

• Place the jack; it should snap in place.
• Flip the board over and solder all the pins.

11. Clean Flux Off Board

If you used no-clean flux, you can skip this step.

Note

This step was written with water-soluble flux in mind.
If you used non-water-soluble flux, you may need to use a different cleaning process.

• Use isopropyl alcohol with a toothbrush to remove the bulk of the flux.
• Clean the board with dish soap and warm water using the same toothbrush.
• Dry the board thoroughly. I like to use compressed air for this step.

12. U1-U2 (Integrated Circuits)

• Carefully bend the pins on the ICs so that they are straight up and down. I usually use a table surface to bend all the leads on one side of the chip at a time.
• Carefully instert the chips into the sockets on the board. Make sure you don't crumple any pins in this process.

13. Finishing Touches

• Insert the screw terminal block (J5) into its header.
• Insert the potentiometer knobs.