The two-channel AD-KEB 20 GX is used to read in digital signals that are present as a contact or active 24V signal. The respective signal state is indicated on the device by means of a red LED. Device communication takes place via the RS485 bus interface and is implemented using the MODBUS RTU protocol.
The power supply and the RS485 bus connection are possible via the rear DIN rail connector. The device is equipped with two rotary coding switches with which the bus address can be set on the device.
An optical search function allows the localization of a single device in a network. For this purpose, the green LED is set in a time-limited flashing mode, between 1 and 255 seconds.
Interface settings as well as the device address can be modified during operation by means of Modbus commands.
All Modbus register addresses and associated commands are listed in the document attachment. The presettable absolute counters continuously add incoming pulses and are saved in a non-volatile memory every hour. The current counters are reset during readout to detect subsets.



Reading in digital signals in general electrical engineering, automation technology and in the water / wastewater sector.
For example: pulse counting with presettable counters or frequency measurement.

Specific characteristics

  • RS485 bus / Modbus RTU protocol
  • Rotary coding switch for bus address setting
  • Counter function
  • Frequency measurement
  • Switching Power Supply
  • Optical search function

Business data

Order number
Rail connector
Artnr: AD-GX-Connector
Zur Durchschleifung von Versorgungsspannung und RS485-Bus
Aufrastbar auf Tragschiene
(DIN EN 50022).
Customs tariff number / country of origin 8543 70 90 / Deutschland


Data sheet pdf en-keb20gx.pdf (211 kB)
Tender text zip (10 kB)
Safety instructions pdf ad-safety-instructions.pdf (43 kB)

Technical specifications

Input voltage 24 V DC / 3,5 mA (max. 30V)
Switching threshold ~ 17 V
Contact load 5 V / 1,5 mA
Frequency < 2200 Hz
Gate time 1 s
Accuracy +/- 0,3%
Counter 0...4294967295
Pulse width (counter) > 25 ms
Counting frequency < 20 Hz
Contact debouncing 20 ms
Counter storage Stündlich
Transmission behaviour
Response time Max. 15 ms
Software protocol Modbus-RTU
Data format 19200, e, 8, 1
Max. bus users 99
Bus termination 120 ohms both sides at the end
Max. length of bus 500 m (no spur lines)
Cable Twisted and shielded
Supply voltage 18 ... 30 V DC
Max. power consumption 800 mW (24V DC)
Dimensions (WxHxD) 6,2 x 92 x 101 mm³
Manner of fastening DIN rail mounting 35mm, EN 50022
Type of protection IP 20
Connection method Screw clamp
Bolting torque terminals 0,5 Nm
Wire cross section Max. 2,5 mm2
Weight ~ 70 g
Environmental conditions
Permissible ambient temperature -10 ... +50 °C
Storage and transport -10 ... +70 °C (no condensation)
Product family standard 1) EN 61326-1
Emission 2) EN 55011, CISPR11 Cl. A, Gr. 1
1) During checking, slight signal deviations are possible.
2) Warning:
This device is not intended to be used in residential areas and can not ensure adequate protection of radio reception in such environments.
Electrical safety requirements
Product family standard EN 61010-1
Galvanic isolation, test voltages
RS485 bus / power supply 1,5 kV, 50 Hz (1 min)
Digital input / Power supply 1.5 kV, 50 Hz (1 min)
Digital input / RS485 bus 1,5 kV, 50 Hz (1 min)
Digital input between themselves 0 kV
Protection circuits
RS485-Bus Electrical surge protection
Power supply Electrical surge and reverse current protection

Block and wiring diagram




CAD / CAE Daten

CAD model is not yet available for this product.

The CAD product overview can be found at:

Display and operating elements

Designation LED Meaning
On green Power supply
E1 red Input 1 state
E2 red Input 2 state
D yellow RS485 Communication
RS485 Address switch(01...99)

Modbus Communication

The AD-KEB 20 GX contains a RS485 bus interface on which the Modbus RTU protocol is used. All control functions of the device can be executed via this bus interface. The preset standard data format is 19200,e,8,1. Adaptation to a different data format is possible at any time. The bus address (1...99) is set to the side-mounted rotary switches. The address 0 is not permitted for the bus operation. However, on this zero position the device is accessible only via the standard data format (19200, e, 8.1). The position 0 thus represents a service position, the example can be used during parameterization error.

The AD-KEB 20 GX supports two Modbus functions. These are the functions "Read Holding Registers" (0x03) and "Write Holding Registers" (0x10). With the "Read Holding Registers" function data can be read from the device and written with "Write Holding Registers" data. The individual register width is 16 bits. Please see the Modbus specification for detailed explanations of the Modbus communication. This is online available for free and can be downloaded from the Adamczewski homepage.

The following Modbus data are accessible via the RS485 bus:

Start address Reg. number Name Datatype [Code] = Value read write
40501 1 Address switch U16 1...99...247 yes yes
40502 1 Flashing function U16 0/1...255 yes yes
40503 1 24V Input 1 U16 0/1 yes no
40504 1 24V Input 2 U16 0/1 yes no
40505 1 Input contact 1 U16 0/1 yes no
40506 1 Input contact 2 U16 0/1 yes no
40551 2 Input frequency 1 FLOAT 0,00...2200 Hz yes no
40553 2 Input frequency 2 FLOAT 0,00...2200 Hz yes no
42901 1 Baud rate U16 Index, see list below yes yes
42902 1 Parity U16 [0]=even; [1]=odd; [2]=no yes yes
42903 1 Input signal type 1 U16 [0]=aktiv; [1]=Contact yes yes
42904 1 Input signal type 2 U16 [0]=aktiv; [1]=Contact yes yes
43001 1 Debouncing 1 U16 x 20 ms yes yes
43002 1 Debouncing 2 U16 x 20 ms yes yes
47209 2 Absolutely counter 1 U32 0...4294967295 yes yes
47211 2 Absolutely counter 2 U32 0...4294967295 yes yes
47213 2 Currently counter 1 U32 0...4294967295 yes no
47215 2 Currently counter 2 U32 0...4294967295 yes no
49102 1 Device reset U16 0 no yes
49105 6 Device type String KEB20GX yes no
49119 1 Firmware version U16 MSB/LSB yes no

Coding baudrate list
index 0 1 2 3 4 5 6 7 8 9
baud 2400 4800 9600 14400 19200 28800 38400 57600 76800 115200

After changing the interface parameters, a device reset is required.

Modbus master mode
The device can even work in Modbus master mode to transfer data between any devices within an RS485 line.
For Modbus master operation, starting with Modbus register 40611, there are 48 consecutive data transfer sets,
with 5 configuration registers each.

Start address Reg. number Name Datatype [Code] = Value read write
40611 1 Source address [1] U16 1...99 yes yes
40612 1 Source register [1] U16 nnnnn yes yes
40613 1 Target address [1] U16 1...98 yes yes
40614 1 Target register [1] U16 nnnnn yes yes
40615 1 Register count [1] U16 1/2 yes yes
40846 1 Source address [48] U16 1...99 yes yes
40847 1 Source register [48] U16 nnnnn yes yes
40848 1 Target address [48] U16 1...98 yes yes
40849 1 Target register [48] U16 nnnnn yes yes
40850 1 Register count [48] U16 1/2 yes yes

For the Modbus master mode, the parameter "Modbus master" must be activated and the address switch set to position 99.
Data transfer begins six seconds after activation or device restart.
A flashing green LED indicates a communication error.

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