Siemens SM 338 User manual

A5E00409892-01

Product Information on the Reference Manual

Programmable Logic Controllers S7-300 Module Data Release 3

1 Position Decoder Module SM 338; POS-INPUT;

(6ES7338-4BC01-0AB0)

Order number

6ES7338-4BC01-0AB0

Characteristics

The position decoder module SM 338; POS-INPUT is distinguished by the

following features:

•3 inputs for the connection of maximum three absolute value encoders (SSI)

and 2 digital inputs to freeze the encoder values

•Direct reaction possible to encoder values in moving systems

•Processing of acquired encoder values of the SM 338 in user program

•Supports clocked operation

•Type of encoder value acquisition (see chapter 1.1.2.1) can be selected:

– Free running

– Clocked

•24 VDC rated input voltage

•Non-isolated against the CPU

•Fast mode selectable; with faster encoder recording and compressed

checkback interface

Fast mode is available as of SM 338; POS-INPUT firmware version V2.0.0 and

as of STEP 7 V5.3+SP2 selectable.

2ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

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Supported encoder types

The following encoder types are supported by the SM 338; POS-INPUT:

•Absolute value encoder (SSI) with 13-bit message length

•Absolute value encoder (SSI) with 21-bit message length

•Absolute value encoder (SSI) with 25-bit message length

Supported data formats

The SM 338; POS-INPUT supports the gray code and binary code data formats.

Firmware update1)

You can use STEP 7 HW Config firmware update to load POS-INPUT in the

operating system memory of the SM 338 in order to extend the functionality and

trouble-shooting.

Note

The old firmware is deleted with the start of the firmware update. If the firmware

update is interrupted or canceled for any reason, the SM 338; POS-INPUT is no

longer functional. Restart the firmware update and wait until it has been

successfully completed.

1) The function is only possible in distributed configuration if the header module (slave interface) supports the

necessary system services

ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

A5E00409892-01

1.1 Synchronous Operation

Warning

The basics of synchronous operation are described in a separate manual.

Hardware requirements

For the synchronous operation of the SM 338, you require:

•CPU which supports clocked operation

•DP master which supports the equidistant bus cycle

•Slave interface (IM 153-x) which supports synchronous operation

Characteristics

Depending on the system parameterization, the SM 338 works in either

non-synchronous or synchronous mode.

In synchronous operation, the data exchange between DP master and SM 338 is

synchronous to the PROFIBUS DP cycle.

In synchronous operation all 16 bytes of the checkback interface are consistent.

If synchronicity is lost due to faults or failure or delay of Global Control (GC), the

SM 338 goes back into synchronous operation in the next cycle without error

response.

If synchronicity is lost, the checkback interface is not updated.

4ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

A5E00409892-01

1.1.1 Terminal Connection Diagram and Block Diagram

Fault indicator – red

L+ 24V

OD (Data)

OC (Clock)

1D (Data)

1C (Clock)

2D (Data)

2C (Clock)

DI 0 (Digital-

DI 1 input)

DC24V (encoder)

M (encoder)

SSI

Short-circuit

protection

Voltage -

monitoring

S7-300 Backplane bus

Logic

RS 422

Twisted pair cables

DI 0

DI 1

Connectionto the

ground of the CPU

Figure 1-1 Module View and Block Diagram of the SM 338; POS-INPUT

Wiring rules

Please observe the following important rules of the wiring of the module:

•The ground of the encoder supply is connected non-isolated to the ground of

the CPU. Thus, connect pin 2 of the SM 338 (M) with low impedance with the

ground of the CPU.

•The encoder lines (pins 3 to 14) must be twisted pairs and shielded. Apply the

shield to both sides.

For the shield connection to the SM 338, use the shield connection element

(order number 6ES7390-5AA00-0AA0).

•If the output current (900 mA) of the encoder supply is exceeded, then you

must connect an external power supply.

ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

A5E00409892-01

1.1.2 Functions of the SM 338; POS INPUT

1.1.2.1 Encoder value acquisition

The absolute value encoder transfers its encoder values in messages to the

SM 338. The transfer of the message is initiated by the SM 338.

•In non-synchronous operation, the encoder values are acquired while it is free

running.

•In synchronous operation the encoder values are acquired synchronized to the

PROFIBUS DP cycle at each Ti.

Free running encoder value acquisition

The SM 338 always initiates the transfer of a message after the end of the

parameterized monoflop time.

Asynchronous to these free running messages, the SM 338 processes the

acquired encoder values during the cycle of its updating rate (see Technical Data).

Thus, in the case of free running encoder value acquisition, encoder values of

different ages result. The difference between the maximum and minimum age is

the jitter (see Technical Data).

Synchronous encoder values acquisition

Synchronous encoder values acquisition is automatically set if, in the DP master

system, the equidistant bus cycle is activated and the DP Slave is synchronized to

the DP cycle.

The SM 338 initiates the transfer of a message in each PROFIBUS DP cycle

at the time Ti.

Synchronous to the PROFIBUS DP cycle, the SM 338 processes the transferred

encoder values.

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1.1.2.2 Gray/Dual Converter

In the Gray setting, the encoder values provided by the absolute value encoder in

gray code is converted into Dual code. In the Dual setting, encoder values

provided by the absolute value encoder remain unchanged.

Warning

If you have selected the Gray setting, the SM 338 always converts the entire

encoder value (13, 21, 25 bits). As a result, preceding special bits affect the

encoder values and following bits could be falsified under certain circumstances.

1.1.2.3 Transferred Encoder Value and Normalization

The transferred encoder value contains the encoder position of the absolute value

encoder. Depending on the encoder used, additional bits which are located before

and after the encoder position are also transferred in addition to the encoder

position.

So that the SM 338 can detect the encoder position, make the following settings:

•Normalization, places (0..12), or

•Normalization, units / revolution

Normalization, places

The normalization determines the position of the encoder values in the checkback

interface.

•If “Places” = 1, 2....12, this indicates that the following non relevant bits in the

encoder values are removed and the encoder value is right justified in the

address range (see following example).

•If “Places” = 0, this indicates that the following bits are retained and available

for evaluation.

This can be useful if you use an absolute value encoder which transfers

information in the following bits (see manufacturer information) and you want to

evaluate these (see also chapter 1.1.2.2).

Parameter units / revolution

A maximum of 13 bits are available for the units/revolution. According to the

“Places” data, the resulting number of units/revolution is automatically displayed.

ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

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Example of normalization of an encoder value

You are using a single-turn encoder with 29units= 512 units/ revolution

(resolution/360°).

In STEP 7 you have set the following parameters:

•Absolute encoder: 13 bits

•Normalization: 4 places

•Units / revolution: 512

Before the normalization: cyclically acquired encoder values 100

310000000000000000000000000XXXX

00100

0000

Transferredbits

Relevant bits

After the normalization: encoder values 100

31 0000000000000000000000000001001100000000

Relevantbits

Double data word

Result: Bits 0 to 3 (4 places, marked with “x”) are omitted.

1.1.2.4 Freeze Function

The freeze function “freezes” the current encoder values of the SM 338. The

freeze function is coupled to the digital inputs DI 0 and DI 1 of the SM 338.

The freeze is triggered by an edge change (rising edge) on DI 0 or DI 1. A frozen

encoder value is identified by the bit 31 (output address) being set. With a digital

input you can freeze one, two or three encoder values.

You must switch on the freeze function, i.e. set the corresponding parameters in

STEP 7. The freeze function is not possible in fast mode.

The encoder values are retained until the freeze function is ended and can thus be

evaluated dependent on the event.

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Ending the freeze function

The freeze function must be ended at every encoder input. You acknowledge the

function in the user program by setting the bit 0, 1 or 2 depending on the channel

with the STEP 7-Operation T PAB “xyz” (for a program example,

see chapter 1.1.4).

After exiting, bit 31 of the corresponding encoder value is again deleted and the

encoder values are again updated. A renewed freezing of the encoder values is

again possible as soon as you have deleted the acknowledgment bit in the output

address of the module.

In synchronous operation the acknowledgment is processed at time To. From this

point in time a renewed freezing of the encoder values can take place via the

digital inputs.

Warning

The freeze function is automatically acknowledged if you newly parameterize the

corresponding channel with different parameters (see 1.1.3 chapter).

If the parameters remain identical, the freeze function remains unaffected.

1.1.3 SM 338; POS-INPUT Parameterization

You parameterize the SM 338; POS-INPUT with STEP 7. You must perform

parameter assignment in STOP mode of the CPU.

As soon as you have set all the parameters, download the parameters from the

programming device to the CPU. On a transition from STOP to ³RUN mode, the

CPU then transfers the parameters to the SM 338.

The parameters cannot be changed by the user program.

ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

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Parameters of the SM 338; POS-INPUT

You will find an overview of the parameters that you can set and their default

settings for the SM 338 in the table below.

The default settings apply if you have not performed parameter assignment in

STEP 7 (default setting bold).

Table 1-1 Parameters of the SM 338; POS-INPUT

Parameter values Range Note

Enable

•Fast mode Yes/no Release parameter.

Affects all 3 channels.

Enable

•Diagnosis interrupt Yes/no Release parameter.

Affects all 3 channels.

Absolute value encoder

(SSI) 1) none; 13 bits; 21 bits; 25 bits none: The encoder input is

switched off.

Code type 1) Gray; Binary Code provided by encoder.

Baud rate 1) ,3) 125 kHz; 250 kHz; 500 kHz; 1 MHz Data transfer rate of the SSI

position decoder. Observe the

relationship between the cable

length and baud rate (see

Technical Data)

Monoflop time 1),2),3) 16 ms; 32 ms; 48 ms; 64 msThe monoflop time is the

minimum time interval between

2 SSI message frames.

The parameterized monoflop

time must always be greater than

the monoflop time of the absolute

value encoder.

Normalization

•Places

•Units / revolution 4) 0to 12

2 to 8192

Normalizing right justifies the

encoder values of the encoder

absolute; non-relevant places are

discarded.

Switching on freeze off; 0; 1 Designation of the digital input

whose rising edge causes a

freezing of the encoder value.

1) See technical data of the absolute value encoder

2) The monoflop time is the time interval between 2 SSI message frames. The parameterized monoflop

time must be greater than the monoflop time of the absolute value encoder (see technical data of the

manufacturer). The time 2 (1 / baud rate) is added to the value parameterized in HW config. At a

baud rate of 125 kHz with a parameterized monoflop time of 16 ms, an effective monoflop time of 32 ms

is actually achieved.

3) The following restriction applies to the monoflop time of the absolute value encoder:

(1 / baud rate) < Monoflop time of the absolute value encoder < 64 ms + 2 (1 / baud rate)

4) to the power of two

Warning

Please note that in non synchronous operation the baud rate and the monoflop

time affect the accuracy and actuality of the encoder values.

In synchronous operation the baud rate and the monoflop time affect the accuracy

of the freeze function.

10 ProductInformation on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3

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1.1.4 SM 338; POS-INPUT Addressing

Data range for the encoder values

The inputs and outputs of the SM 338 are addressed as of the initial module

address. The input and output address is determined at the configuration of the

SM 338 in STEP 7.

Input Addresses

Table 1-2 SM 338; POS-INPUT: Input Addresses

Encoder input Input address (from the configuration) + address offset

0”Initial module address”

1”Module start address” + 4 bytes address offset

2”Module start address” + 8 bytes address offset

Structure of the double data word in Standard Mode

For each encoder input the double data word is made up as follows:

Freeze

0 = encoder value is not frozen. The value is constantly updated.

1 = encoder value is frozen. The value remains constant until

acknowledgment.

25 bit sensor value in gray or binary code

31 024

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