Brother HL-4000CN Service Manual page 301

1.3 Signal Name
Signal Name
Signal name configuration
REGI SENSOR
Part name
This signal is L when paper is detected at the registration sensor but H (+5 VDC)
when not.
1.4 DC Voltage
Unless otherwise specified in a note or other, the DC voltage is measured between
each test point and the frame and its value is within the following ranges:
+5VDC (H)
(L)
+12VDC (H)
(L)
+24VDC (H)
(L)
1.5 Input/Output Test
DI: Digital input test
DO: Digital output test
The DI/DO diagnosis can be executed from the control panel.
SENSED (L)
Status Logical value
+4.5 to +5.3VDC
0.0 to +1.0VDC
+10.0 to +13.2VDC
0.0 to +1.5VDC
+22.0 to +25.7VDC
0.0 to +3.0VDC
VI-4
+5VDC
Voltage at signal level H