Whatever a controller port's auxiliary pin can supply, the manual for your exact controller and port prints the number; 0.1 to 3 A and a remembered 200 mA are figures for other equipment. List each accessory fed from the pin, add their rated currents, and keep the sum under the printed limit. When the sum will not fit, move the largest accessory to its own supply instead of overloading the pin.
Power a small accessory from a controller port's 5 V pin and one question decides the design: how many milliamps does that pin actually carry? The manual for your exact controller, port and revision prints the number. Models differ enough that a figure quoted for another controller sizes nothing. The method around that number never changes: list every device wired to the pin, add their rated supply currents, and keep the sum under the printed limit. This checklist walks through finding the limit, counting the loads, working a real budget, and moving an accessory to its own supply when the pin runs out.
What does the auxiliary pin on a controller port actually supply?
The 5 V pin on a serial or communication port is a vendor-added auxiliary supply. It lets you power a small accessory, such as an operator terminal or a communications module, from the controller itself instead of from a separate power supply.
The standard behind the common serial port governs the signaling voltages on the data lines and says nothing about powering accessories. The pin is the controller maker's addition, and its limit lives only in the controller's documentation. That is also why the number is easy to miss: it sits in the terminal description or electrical specifications for your model, not on a features page. Treat it as a convenience for small loads. Plan from the first sketch for anything bigger to bring its own supply.
How much current can you draw from a controller port's pin?
Budget against the figure printed for your exact controller and port: one controller family's published technical data documents an output bus current capacity of 0.1 to 3 A at 5 V DC depending on the model, so a number quoted for one model cannot size another.
Look in three places, in this order: the hardware manual for your exact model and revision, under the port's electrical specifications or terminal description; the controller's system power budget, because the pin is fed from the controller's own supply and every milliamp drawn there loads that budget too; and the accessory datasheets, which print supply current at the pin's voltage for each device.
The 200 mA you will hear quoted for a legacy fixed-I/O PLC's serial-port pin is a lead, not a rating. It becomes a design limit on the day the manual for your CPU prints it. A figure recited from memory or borrowed from another model carries no weight.
Which devices count against the current budget?
Count every device that actually receives its power from the port's pin. Add each one's rated supply current at that voltage, and leave out anything fed from its own supply, because a self-powered accessory adds communication load but no current to the pin.
Walk the power path on the wiring diagram before you trust the communication path:
- An operator interface or text terminal wired to the port's supply and return counts in full, at the current printed in its datasheet.
- A protocol converter or network module powered from the pin counts the same way.
- A sensor or reader fed from the pin counts too, even though its cable also carries signal.
- Anything with its own power feed stays out of the budget; the pin sees communication only.
Two habits keep the count honest. Check the accessory's documentation for whether its cable passes power, because a communication cable is not always a power cable. And give the accessory you plan to add next year a line in the budget with the same weight as the ones already mounted.
What does a worked example of a current budget look like?
A controller whose manual documents 200 mA on the serial port's pin, feeding a 140 mA terminal and a 50 mA converter, carries 190 mA with 10 mA to spare; a 350 mA terminal on the same pin is 200 mA over.
Two accessories hang off the pin, and their datasheets, for this example, read 140 mA for a text operator terminal and 50 mA for a protocol converter.
- 140 mA + 50 mA = 190 mA
- 190 mA against the 200 mA limit fits, with 10 mA spare
- 5 V × 0.19 A = 0.95 W drawn through the pin
Now swap the text terminal for a color graphic terminal whose datasheet reads 350 mA:
- 350 mA + 50 mA = 400 mA
- 400 mA against the printed limit is twice the capacity, so the graphic terminal stays off the pin
- The graphic terminal takes a regulated supply of its own and shares only the signal lines with the port
The example figures are placeholders. The numbers that matter are the rows printed in your controller manual and your accessory datasheets, and the arithmetic is identical whatever they read.
When should an accessory get its own supply instead?
Give an accessory its own supply when its rated current would push the port total past the printed limit. Wire that supply as the replacement for the port's pin, never in parallel with it, because two supplies tied to one rail drive current into each other.
Three conditions decide it:
- The budget fails: the example's 400 mA against a 200 mA limit ends the discussion.
- The documentation permits it: the accessory's wiring instructions must show an external-supply arrangement and how its signal common connects.
- The replacement is complete: the port's supply conductor stops feeding the accessory, and only the signal lines stay on the port's connector.
Never tie a second 5 V output to the pin. Two supplies on one rail push current into each other, and the fight shows up as unexplained resets or a warm regulator. If the accessory's documentation shows no external-supply arrangement, the fix is a smaller accessory or a second interface path, the way you would add another operator interface when the PLC ports are full.
A separate supply also buys margin against dips. A rail shared with inductive loads sags while the loads switch, which is the mechanism behind an HMI that freezes when it shares a DC supply with solenoid coils.
Recommendation: size the pin against the figure printed in the manual for your exact controller and port revision, keep the sum of every pin-fed accessory at or below that figure, and when an accessory pushes the total over, give the accessory its own regulated 5 V supply wired for signal common exactly as its instructions show. Treat a remembered 200 mA as a pointer to the specification, and let the printed number close the question.
How do you check the wiring before you power up?
Verify the wiring with the controller powered down: confirm the supply and return terminals from the manual, confirm the accessory's input polarity and current from its own datasheet, and confirm the calculation against the printed limit before the first connection carries current.
- Isolate power to the controller and to every accessory before the first conductor moves.
- Mark the supply and return terminals for the exact port from the controller manual's connector drawing; the figure you verified applies to that port on that model only.
- Mark the accessory's supply input and polarity from its own datasheet, and check whether its cable passes power or only signals.
- Re-add the rated currents of every device on the pin and set the total against the printed limit and the controller's power budget.
- Energize once, check that the accessory starts and communicates, and file the calculation with the drawing set as the acceptance record for the installation.
One safety note: make connections on a dead connector. The supply pin sits beside signal pins, and a slipped probe that shorts it onto an input on a live connector can cost more than the accessory.
What should you send with a quote?
Send the controller model and manual revision, each accessory's model and rated current at the pin's voltage, the calculated port total, and, when the project includes a notice screen, the viewing distance, mounting surface and how many hours a day the screen is on.
Grouped for whoever prices the job:
- Controller model, port and manual revision, so the printed limit can be checked against the exact hardware
- Each accessory model with its rated supply current, plus the port total you calculated
- For a notice or status screen on the same project: viewing distance from the farthest reader, the mounting surface, the power available at that spot, and how many hours a day the screen is on
Our shop-floor screens cover this kind of floor work, and the quote confirms ports, power and mounting for the model that fits.
Questions
Why is there no current limit in the RS-232 standard?
RS-232 is a signaling standard: it sets the voltage levels on the data lines, with driver outputs up to about ±25 V, and it makes no provision for powering accessories. An auxiliary power pin is the controller maker's addition, so its current limit comes from the controller manual, never from the standard.
An accessory datasheet lists power in watts. How do I get milliamps?
Divide the watts by the supply voltage: current in amps equals power in watts divided by voltage in volts. A terminal rated at 1.25 W draws 1.25 ÷ 5 = 0.25 A, which is 250 mA, and that is the number added to the port budget. Where a datasheet prints both a typical and a maximum current, budget the maximum.
What actually happens when the load exceeds the port's limit?
It depends on what the manufacturer built in. One controller family's technical data documents short-circuit protection and an internal fuse on its auxiliary bus, so an overcurrent trips the protection and the fuse then needs service. Where no protection is documented, an overload can drag the rail down until accessories reset or the controller misbehaves. The printed limit stays the design number, and protection is the backup you want to keep unused.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.