Beckhoff EL3255 Precise load cell input terminal (RMB)

The EL3356 is a 16-bit and the EL3356-0010 a 24-bit with Distributed clocks input terminal which enables direct connection to a 4- or 6-wire strain guage. It is a highly accurate factory calibrated measurement device.

This input terminal has a vast configuration scope and it is imperitive to consult the documentation.

The slave can also be used as a dual ADC instead of the RMB mode (resistance measurement bridge) with input range of ca. 13V and 27mV. This mode is not calibrated by the manufacturor. (Only the bridge measurement mode RMB is calibrated).

The following PDO's are used:

Block inputs are the binary control inputs.

  1. Start calibration
  2. Disable calibration
  3. Input freeze
  4. Sample Mode (EL3356-0010 only)
  5. Tara

Block outputs:

Status port in RMB Mode:

  1. Overrange
  2. Data invalid
  3. Error
  4. Calibration in progress
  5. Steady state

Status port in ADC Mode:

  1. Underrange
  2. Overrange
  3. Error

Apart from the common slave options, the following options can configure the slave.

2 ch ADC only

The terminal is used as a dual ADC, instead of default RMB mode.

RMB Range [mV/V]

Enter a value for full scale, e.g. "4.0". A gain is calculated so that +-MAXINT appears on the raw output port when the RMB reaches this value. The gain is written to SDO #x8000:21 and thus overwrite any previously (permanently) stored value upon startup.

Leaving this value empty ("[]") will leave the SDO untouched.

Distributed clock

Only available for EL3356-0010

CoE Configuration

Here the slave is configured according to the documentation.

Note: Configuring Dynamic filter delta #x8000:14 This is a real32 value, while the mask input can only accept integer values. To convert, use matlab's num2hex() function:
e.g. hex2dec(num2hex(single(20.0)))

RMB scaling factor

When using the slave in RMB mode, it has a generous selection of gains and offsets that it uses to calculate the output value which makes it quite confusing to configure. A careful study of the calculation algorithm is required.

Moreover, the values are stored in non-volatile memory on the slave. This enables the user to calibrate the slave with sensor data (SDOs #x8000:[23,25]) and thus get a consistent output. The disadvantage is that the data needs to be reconfigured every time the slave is replaced.

When the RMB Range [mv/V] mask configuration option is used, a value for SDO #x8000:21 (Gain) is calculated and written, overwriting any previously stored value. The gain is calculated as follows:

Gain = 2.0/5.0/1000.0*2^31/range
When using RMB Range, it is thus recommended that all SDOs involved in the calculation (especially #x8000:[24,26,27]) to be at their factory default values (see the documentation), and #x8000:[23,25] close to them.