Camera Link range extender - System Latency Specification
KY-EXT-CL: System Latency Specification
The KY-EXT-CL system carries Camera Link video over fiber using two units. Latency is the time from the first data at the Camera Link input of the Device unit to the first data at the Camera Link output of the Host unit.
All values are for 8-bit pixels.
System units
| Unit | Function |
| Device unit | Camera Link → Fiber |
| Host unit | Fiber → Camera Link |
Latency formula
Total latency (µs) = N / (T × F) + N / 1250 + 0.7 + Cable delay
| Symbol | Meaning |
| N | The image line length in pixels or 8000, whichever is smaller |
| T | Number of taps |
| F | Camera Link pixel clock in MHz |
| 1250 | Data rate of the fiber link between the units, in bytes per µs |
| 0.7 | Internal processing time of both units, in µs |
| Cable delay | About 5 µs per km of fiber |
When the Device unit sends data: the Device unit collects image data from the camera into a packet and sends the packet over the fiber when one of these happens, whichever comes first:
- the packet reaches 8000 pixels(bytes), or
- the current image line ends.
Long lines are therefore sent in packets of 8000 pixels(bytes), with the rest of the line sent at its end. A line shorter than 8000 pixels(bytes) is sent as one packet when it ends. Data never waits for the next line or the next frame. The time to collect one packet is N / (T × F), the first part of the formula.
When the Host unit outputs data: the Host unit stores each packet it receives from the fiber. When the whole packet has arrived and been checked, the Host unit immediately starts sending it on its Camera Link output. The output uses the same Camera Link mode, number of taps and pixel clock as the camera. The time to receive one packet is N / 1250, the second part of the formula.
Example: Base mode, 2 taps, 85 MHz pixel clock, line length 2048 pixels, 100 m fiber. Total latency = 2048 / (2 × 85) + 2048 / 1250 + 0.7 + 0.5 = 12.0 + 1.6 + 0.7 + 0.5 = 14.9 µs
Table 1: Total latency for long image lines (8000 pixels or more per line)
| Mode | Taps | 20 MHz (µs) | 40 MHz (µs) | 66 MHz (µs) | 85 MHz (µs) |
| Base | 1 | 407.1 | 207.1 | 128.3 | 101.2 |
| Base | 2 | 207.1 | 107.1 | 67.7 | 54.2 |
| Base | 3 | 140.4 | 73.8 | 47.5 | 38.5 |
| Medium | 4 | 107.1 | 57.1 | 37.4 | 30.6 |
| Medium | 6 | 73.8 | 40.4 | 27.3 | 22.8 |
| Full | 8 | 57.1 | 32.1 | 22.3 | 18.9 |
| 80-bit | 10 | 47.1 | 27.1 | 19.2 | 16.5 |
Table 2: Total latency for shorter image lines (at 85 MHz pixel clock)
| Mode | Taps | 1024 pixels (µs) | 2048 pixels (µs) | 4096 pixels (µs) |
| Base | 1 | 13.6 | 26.4 | 52.2 |
| Base | 2 | 7.5 | 14.4 | 28.1 |
| Base | 3 | 5.5 | 10.4 | 20.0 |
| Medium | 4 | 4.5 | 8.4 | 16.0 |
| Medium | 6 | 3.5 | 6.4 | 12.0 |
| Full | 8 | 3.0 | 5.4 | 10.0 |
| 80-bit | 10 | 2.7 | 4.7 | 8.8 |
Notes
- Table values do not include cable delay. Add about 5 µs per km of fiber.
- Latency depends on the Camera Link mode, number of taps, pixel clock frequency and image line length.
- For pixel clocks lower than 85 MHz, use the formula above to calculate Table 2 values.
- Latency does not depend on frame size or frame rate.