Bind a text panel to the counter's current-value register when the panel will also show messages, or drive an electromechanical counter from one spare PLC output when one number is all the operator needs. Size the digits from the largest value the counter can reach: 5 digits for a 16-bit counter, about 0.75 in of digit height for a 15 ft glance. At one count every 6 seconds, speed rules out none of the paths.
A part count that lives only inside the controller is a number nobody can use. Two paths cover almost every counter display job: bind a numeric field on a text or touch panel to the counter's current-value register, or pulse an electromechanical counter from one spare output point. Seven-segment LED wiring and a serial LCD fill the corners where cost or spare ports decide. There is also a fifth answer, no display at all, when the count changes no decision. Which path fits comes down to four numbers: the largest value the counter can reach, the distance the operator reads from, how fast the counts arrive, and whether an output point is genuinely spare. At one count every 6 s, speed is settled for every path. This article sizes each choice and ends with what to ask for before you buy.
What decides the choice
Four questions settle it.
How large can the count get? The register width plus your reset policy set the digit count, and the count you happen to see this week sets nothing.
How far away is the reader? That sets digit height.
Is there a spare output point? The pulse-driven paths need one; the communications paths need none.
Does the count change any decision on the floor? If nobody acts on it, buy nothing and stop.
One placement rule sits above all four: a display built into the controller travels with the controller. A unit mounted on the controller in a back room serves nobody standing at the line, so the display goes where the eyes are, even when that means a remote path.
Size the display from the largest value the counter can reach
Start from the register, then apply the reset policy.
A 16-bit counter register tops out at 32,767 counts (65,535 if unsigned); a 32-bit register reaches 2,147,483,647. That is 5 digits and 10 digits of display. Both widths exist in common controllers, so confirm the width in the counter documentation before you order hardware.
The reset policy usually buys digits back. A counter reset at the start of each shift can never exceed one shift's maximum production, which the example below works out at 4,800 counts, a 4-digit number. The display still has to survive the day a reset gets skipped, so the safe specification is the register ceiling: 5 digits for a 16-bit counter, and 10 only for a 32-bit counter that never resets.
Electromechanical counters are bought by digit count and roll over silently at their limit, so check the digit roll against that ceiling. A panel field costs screen space and no hardware. A seven-segment assembly costs output points per digit, which the table below counts. If the count carries over between shifts or a reset needs care, save the previous count before a PLC counter reset walks through keeping the old total.
Give the digits enough height for the distance
The legibility planning figures are 30 ft of viewing distance per inch of capital height, and 20 ft per inch when the read must be fast, which a glance up from a running machine is (FHWA planning figures). At a 15 ft reading distance that is a 0.5 in floor and a 0.75 in working size: 15 / 30 = 0.5, 15 / 20 = 0.75. Choose 0.75 in of digit height for an operator who is meant to keep working while reading.
Digit height is a quoted specification on seven-segment and electromechanical counters. On a panel it is the text size you assign to the numeric field, so the same arithmetic applies at no hardware cost. The same numbers size any floor screen: what size digital signage screen do you need carries the full table, including the 1.7 x capital-height line spacing for multi-line text.
The four display paths
Read the needs column against the spare points and ports you actually have, and one row will drop out.
| Path | What it needs | Best when | Confirm before you buy |
|---|---|---|---|
| Text or touch panel on the counter tag | A communications port and the counter's current-value register, shown in a numeric display element | The panel will also carry targets, alarms or messages | The panel's driver list includes your controller, and the register address |
| Pulse-driven electromechanical counter | One spare discrete output that pulses once per part | One number is all the operator needs and the unit can hang on the cabinet or a post | Input voltage against the output type, own supply or external, digit count |
| Seven-segment LED display | Output points: 7 to 8 per digit driven directly, 4 per digit with a BCD input | Upfront cost matters more than wiring time | Total spare points after existing machine functions, and input voltage |
| Serial LCD on RS-232 | A spare serial port and a message instruction that sends the value as text | The controller has an unused serial port and you accept formatting work | The display's command set (clear screen, cursor position) and an enclosure, because bare glass ships without one |
| No display | Nothing | The count changes no decision on the floor | Nothing; this is a legitimate answer |
Wire any pulse path from a genuinely spare point with the cabinet locked out; an output that already drives a machine function is not spare, whatever the display saves.
The panel path is the same move as showing motor run hours on an HMI from a PLC counter: point a display element at a register and let the controller keep the count.
Keep the count read-only
A numeric display element reads the register and writes nothing. Writes come from edit elements, and that is where a counter display grows into a control station. In the panel setup, disable write access to the counter register, and switch on the panel's password protection where it offers one. Some panels also block every write while the controller is in Run, which suits a display-only screen exactly. Do not strip a keypad off the unit as the access policy; configuration does the job and can be undone at the next commissioning. For the wider pattern, specify a read-only process display for a plant keeps a floor screen display-only on purpose.
Count rate: when six seconds settles it
One count every 6 s is 10 counts per minute; at the fastest interval worth planning around, one every 3 s, it is 20 per minute. Every path in the table keeps up at those rates. A relay or transistor output switches far faster than 20 times a minute. A panel field polled at 1 s never shows a value more than a second stale at 20 updates a minute, so set the poll interval to 1 s and leave it.
Speed becomes a selection question only above a few tens of counts per second. Scan times in ordinary controllers run 1 to 10 ms, and a pulse that arrives and clears inside one scan is lost to scan-based counting; that is the job of a high-speed counter input. A 6 s interval sits about 600 times slower than a 10 ms scan (6,000 ms / 10 ms = 600), so ordinary counting sees every pulse. PLC up down counter: scan timing and direction logic covers how scan order shapes counting behavior.
Example: sizing a counter display for a two-shift cell
Scenario: a machine counts finished parts, one every 6 s at full rate, and runs two 8-hour shifts a day. The counter is 16-bit and resets at the start of each shift. The operator reads the display from across the cell, 15 ft away.
Largest count per shift: 8 h x 3,600 s/h = 28,800 s per shift. At one part every 6 s that ceiling is 28,800 / 6 = 4,800 counts, and that assumes the machine counts during every second of the shift. Four digits hold 4,800, but the 16-bit register allows 32,767 if a reset is ever skipped, so specify 5 digits once and both cases are covered.
Digit height: the read is a glance from 15 ft, so the fast-reading figure applies: 15 ft at 20 ft per inch = 0.75 in digits. The 30 ft-per-inch floor is 15 / 30 = 0.5 in, which forces a deliberate stare at a moving job, so 0.75 in it is.
Pulse-path check: two shifts give 2 x 4,800 = 9,600 counts a day. At 240 production days a year that is 9,600 x 240 = 2,304,000 output operations a year. Where the counter accepts one, a transistor output takes mechanical wear off the table. A relay output at 2.3 million operations a year still sits inside the mechanical life commonly quoted for plug-in relays, in the millions of operations, but the transistor ends the discussion.
Result: a 5-digit display with 0.75 in digits, fed either by a panel field polled at 1 s or by a transistor output pulsing an electromechanical counter.
When to pick something else, and what to ask for
When another path fits better:
- No spare output point: drop the pulse-driven counter and take a communications path (panel or serial), which run on ports, not points.
- Washdown or coolant spray at the spot: mount the display out of the spray, behind the machine's window or in the operator station. Where it must live in the spray, buy a unit the maker publishes a washdown rating for and follow its gasket and installation instructions to the letter.
- The controller sits in a back room: any display mounted on the controller is as far away as the controller. Choose a remote path: a panel on the network, pulse wiring to the line, or a serial run.
- Operators also need targets, andon states or downtime reasons: a single number has stopped being the job. A panel is the choice, and the counter is one field on it.
What to ask for before you buy:
- the counter register's width and address, from the controller's counter documentation
- the reset policy and the largest value it allows
- the reading distance, and whether the read is a glance or a study
- the count interval at full rate
- spare output points and their type, relay or transistor
- for a panel: the driver for your controller and the poll interval you will set
- the conditions at the mounting spot, including washdown
- power at the spot: an outlet, or a low-voltage feed from the cabinet
Recommendation: If the panel will also show targets, messages or alarms, bind a numeric display element to the counter's current-value register, set its poll to 1 s, and disable writes to that register in the panel setup. If one number is genuinely all the operator needs and the cabinet has a spare point, drive an electromechanical counter from a transistor output, sized at 1 in of digit height per 20 ft of reading distance. If the count changes no decision, buy nothing and leave it in the controller.
Where a shop-floor signage screen fits
The live register is the job of the paths above. A signage screen plays loaded content offline, and the built-in player has no controller driver, so a live count stays outside its reach. Where the screen earns its place is the board beside the counter: shift target, quality holds, the safety topic of the week, built as slides and refreshed when the plan changes. If you want the live value on that screen later, the route is your own Android app, which our shop-floor signage accepts. Mount it on a fixed bracket close to the wall, and power it from the cabinet feed through a DIN-rail supply where the corner has no outlet. Our shop-floor signage is built for those corners; if the board grows, the display range covers wall and counter sizes quoted per project.
Questions
Does the counter display need to match the controller's brand?
Only the panel path cares. A text or touch panel talks the controller's protocol, so buy one whose driver list names your controller and bind it to the counter's current-value register. The pulse counter and the seven-segment paths are protocol-free: a discrete output pulse is the same signal whichever controller computes it, and a serial LCD needs only a port that sends text.
What happens to the displayed count after a power cut?
Counter instructions in ladder logic are retentive: the accumulated value survives a power cut and clears only on a reset, so the display comes back with the last count. Check the manual for the rare exceptions and for controllers where you must place the counter in retentive memory.
Can the counter display count parts straight from the sensor, with no controller in the loop?
Yes, and it frees an output point. An electromechanical or electronic counter takes a proximity or photoelectric sensor straight into its count input when the input matches the sensor's output type and voltage, so the display counts whether or not the controller is watching. The trade is that the count then lives only in the display: batching, recipes and downtime logic never see it. Keep the count in the controller when anything downstream uses it.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.