Problems with Analog Output of S7-200 EM 235 Analog Module
The analog output of the EM 235 may be driven to -10 VDC when the module is powered up. Siemens recommends that all S7-200 systems using the EM 235 module (part number 6ES7 235-0KD22-0XA0) must implement the remedies contained in this SIMATIC Update if the +/-10 VDC analog voltage output signal is used . The problem can only occur on a system using a S7-200 CPU and EM 235 module (6ES7 235-0KD22-0XA0). The older EM 235 modules (6ES7 235-0KD20-0XA0 and 6ES7 235-0KD21-0XA0) do not exhibit the specified condition.
When the EM 235 Analog Module powers up, the +/-10 VDC output of the module is set to -10 VDC until the analog output is cleared (see Solutions - Option #1 below on how the output is cleared). The error can occur on any system using a S7-200 CPU and a 6ES7 235-0KD22-0XA0 EM 235 module.
The EM235 module provides an analog output channel that allows the user to select either a +/-10 VDC analog output signal or a 0-to-20-ma analog output signal. The error occurs only on the +/-10 VDC analog voltage output signal and does not affect the 0-to-20-ma analog output signal. Siemens recommends that all S7-200 systems using the EM 235 module (part number 6ES7 235-0KD22-0XA0) must implement the remedies contained in this SIMATIC Update if the +/-10 VDC analog voltage output signal is used .
Three potential solutions are listed below. Each potential solution needs to be evaluated by the customer to determine which solution can best be implemented. If none of the solutions listed below can be implemented, then the EM 235 module must not be used to control devices needing a +/- 10 VDC control signal.
Note: The systems that are affected by this module error are those systems that utilize the full voltage range of the analog output (-10 VDC to + 10 VDC). If your system only requires a 0 to 10 VDC signal, then this error should not affect your application.
Option 1: If the external device being controlled has an "enable" input the following sequence of operations should be performed. The device's enable input should be "enabled" AFTER the CPU's logic has initially written to the analog output of the EM 235 module. Writing to the analog output before the external device is enabled will clear the error in the module. After the analog output error is cleared the EM235 output voltage will follow the user program.
Option 2: If your device being controlled does not have an "enable" input and a -10 VDC signal could cause an unwanted effect in your system, then Siemens recommends that an EM 232 Analog Output module be utilized in place of the EM 235's analog output. The two channels of the EM 232 provide the same functionality as the analog output of the EM 235 module, but without the error condition.
Option 3: If the device being controlled can utilize a 0-to-20-ma control signal, then Siemens recommends that this control signal be utilized. There is no error in the operation of the EM235's 0-to-20 ma current signal.
Reverse Voltage Protection:
If the device you are controlling could be damaged by a -10 VDC signal, Siemens recommends that a clamping diode be installed between the [M] and [V] screw points on the EM 235 module. This will clamp the -10 VDC output of the EM 235 module. See the following diagram.
The problem can only occur on a system using a S7-200 CPU and EM 235 module ( 6ES7 235-0KD22-0XA0). The older EM 235 modules ( 6ES7 235-0KD20-0XA0 and 6ES7 235-0KD21-0XA0) do not exhibit the specified condition.
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