Design and manufacture of an RS232 isolation board driving 24-volt outputs for a gauge and workpiece measurement system manufacturer

How the brief came about

The challenge in this case was to let a computer control 24 V devices directly from a program. The customer is a maker of gauges and workpiece measuring systems. The usage example in the board's product documentation is controlling a status light when a measured workpiece fails.

Many measuring or inspection machines use a computer to decide the result, but that result has to appear as something operators can see immediately, such as a status light. These devices commonly run on 24 V, which a computer's RS232 port cannot supply itself, so an intermediary board is needed to receive commands and drive the outputs instead.

Requirements and constraints

The board's requirements come from the product documentation. The board receives commands via RS232 and uses a single 5 V supply, then generates 24–26 V on the board itself. The specification is as follows.

The limitation to be aware of is current. The 3 A figure is a per-channel rating according to the product documentation, while the current actually available when several channels are on at the same time depends on the power supply. Before using several channels at once, the total load must be checked against the power supply.

How the IDB9 board receives commands from a computer and drives 24 V

The IDB9 board receives commands from the computer through the RS232 port, passes the command signal through an optocoupler, and has MOSFETs switch each output channel on and off. The 24–26 V for the outputs comes from the DC-DC converter circuit on the board, so the computer only sends commands and does not have to power the devices itself.

1. 1

   The computer sends commands via RS232

   The computer connects to the board with a cable into the DB9 connector, an RS232 serial port commonly found on industrial computers.

2. 2

   The command signal passes through an optocoupler

   The command signal passes through an optocoupler (a chip that transfers a signal between circuits using light) before reaching the output driver circuit.

3. 3

   MOSFETs switch 5 output channels on and off

   Each MOSFET (an electronic switch with no mechanical contacts) controls one output channel.

4. 4

   The DC-DC circuit generates 24–26 V on the board

   The board uses a 5 V supply, and the DC-DC converter circuit on the board steps the voltage up to 24–26 V for the outputs.

5. 5

   Wiring devices at the screw terminals

   Wires from status lights or end devices connect to the screw terminal connectors (Terminal Block) on the edge of the board, and can be removed and replaced without soldering.

An intermediary board like this gives a clear division of roles. The program on the computer only sends commands through a communication port, while powering the devices is handled by the driver circuit on the board. Boards of the same specification can also be ordered in batches, instead of wiring up circuits on site one machine at a time.

Deliverables

What we deliver is the IDB9 board, in the quantity on the purchase order for each batch. The boards are manufactured from the circuit and printed circuit board (PCB) design we created in 2016.

We also prepared product documentation for the board. It states the specification in the table above and gives an example of use: controlling a status light.

How the customer reordered this board model

The customer reordered the same IDB9 board in batches, with purchase order or delivery documents from 2018, 2020 and 2022, and no redesign was needed. The sequence according to the documents is as follows.

1. 2016: We designed the circuit and PCB of the IDB9 board
2. 2018: Purchase order for 10 sets of the board
3. 2020: Project summary records delivery of all 22 sets
4. 2022: The customer ordered more; the batch delivered in November was 10 sets

The documents do not state whether the quantities in each period overlap, so we have not added the figures into a single total.

Results confirmed by purchase orders and deliveries

The confirmed result of this case is the repeat orders. This maker of gauges and workpiece measuring systems reordered the same IDB9 board model from 2018 to 2022.

We delivered 10 sets of the IDB9 board in November 2022 and issued receipts for all 10 sets in December 2022. This case has no information on how the boards performed on the customer's machines, so we report only the documented purchase orders and deliveries.

What we learned

This work offers 3 principles for designing boards that receive commands from a computer and drive devices in machinery.

• Having an optocoupler on the board does not mean the whole system has isolated grounds. If the job requires ground isolation or surge protection, it must be written as a requirement from the start, and both the signal path and the power supply circuit must be checked.
• The per-channel current rating and the power supply's capacity are two different values. Product documentation must separate them clearly, and the actual load must be checked when several channels are on at the same time.
• A board that customers reorder needs its circuit design, parts list and product documentation kept together as one set. In this work, the board model was designed in 2016 and the customer kept reordering it up to 2022.

How this approach can apply to your work

If your computer or program needs to control a status light or a 24 V device, you can use the same approach as this case: the computer sends commands and an intermediary board handles driving the outputs. What needs adapting depends on the control interface, the number of channels and the actual load of the end devices.

• Summarise the number of output channels and the voltage and current the end devices actually use, including how many channels must be on at the same time.
• Specify the channel the computer or controller will use to issue commands, and the command format the program will send.
• State the board's power source, the size that will fit, and the mounting position in the cabinet or machine.
• Estimate the quantity per batch and the reorder plan, so the circuit design and parts list can be kept for the next batch.