Before using this driver in the PROMOTIC application it is highly recommended to watch "
PROMOTIC video tutorial 4 - Communication drivers".
Basic properties of the driver:
- Usage of this driver requires purchase of the
PmChar licence. When developing the application in the freeware mode
PmFree, or with
development environment and while testing it in runtime, this component is always functional.
- The communication is done
for Ethernet (PROMOTIC application is client) or
for serial link (
COM1,
COM2 ...) (for Ethernet-server the the
PmCharServer communication driver is used).
The driver is simple but powerful tool for communication by
user configured protocol. It is determined first of all for the communication where using a special communication protocol doesn't pay in the end.
The usage of the driver is especialy handy for:
- communications by simple protocols.
- both text and binary oriented communications.
- testing purposes.
The driver allows using both message
of the Master type and
of the Slave type.
Slave messages for Ethernet:
- Even in this case the driver establishes connection (i.e. it is a client). Slave messages start to wait for receipt after the connection with the server is established.
By means of the
PmChar driver it is possible to communicate with the total range of devices and PLC's whose protocol is simple and its description is known, for example:
- Dataloggers of the OMEGA, ESC, .. company
- and many others
The
PmChar driver is not determined for the emulation of more complex protocols (that require for example precise keeping the timing on the communication line, ongoing non-standard control of modem signals, etc.).
We'll be glad
to help you with the decision if the given protocol can be implemented by the
PmChar driver; don't hesitate to contact us! (
sw-support@microsys.cz).
Recommended values of the PmaComm object parameters:
Description and recommended values for the Protocol parameters:
| Response receipt timeout | Only for serial link. It has sense only for messages of the Master type. The time (in milliseconds) the driver is waiting for the response on sending the message.
If no response comes during this time, then the transmission of the message is terminated (the onEndOfTransfer event is triggered with error 24 or 66). |
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| Received data will always be in only one packet | Only for Ethernet.
If checked, then it is assumed that the entire content of the received message will always fit into a single IP packet.
This setting affects the optimization of the reception rate if ETX cannot be used:
Once the packet is received the reception is terminated and the system does not wait for timeout (defined in the "Timeout of one packet transfer [ms]" configurator).
The packet size is predefined by current network. It can be e.g. 521 bytes, 1024 bytes etc. |
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| Use STX | Specifies whether the protocol uses a special character for starting the transmission.
If the usage of the character for starting the transmission is enabled, then it is put automatically before the user data prepared for the transmission. On the other hand the fact if the character for starting the transmission finds itself at the beginning, is tested for each data receipt. If it does not, the communication message is terminated with an error. |
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| STX value | Specifies the ASCII value of the character for starting the transmission (character has usually the value of 2). |
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| Use ETX | Specifies whether the protocol uses a special character for ending the transmission.
If the usage of the char for ending the transmission is enabled, then it is put automatically after the user data prepared for the transmission. On the other hand the fact if the character for ending the transmission finds itself at the end, is tested for each data receipt. If it does not, the communication message is terminated with an error.
The usage of the ETX signal optimizes the speed rate of the receipt if a message with a variable length is received (to the value of the String type). Immediately after receiving the ETX signal, the system regards the message as completed (eventually it expects only check sum characters) and it ends the receipt instead of waiting for the timeout between receiving two characters. |
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| ETX value | Specifies the ASCII values of the character for ending the transmission (character has usually the value of 3). |
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| Checksum | Specifies the method of the check sum calculation:
- none: communication without the check sum.
- XOR 1 byte: The checksum of the XOR type is used (it goes about so-called vertical parity). It has the size of 1 byte and mathematically it is the XOR of all bytes that are included into the check sum.
- SUM 1 byte: The checksum of the sum type is used. It has the size of 1 byte and mathematically it is the arithmetic sum of all bytes that are included into the check sum (cut to 1 byte).
- SUM 2 byte: the same as "SUM 1 byte" but it has the size of 2 bytes (cut to 2 bytes).
- CRC 2 byte CCITT [polynom 0x1021 = x16+x12+x5+1, use SBUS]: ( Cyclical Redundancy Check) - Special checksum, that is used for example in PmSBUS communication. - CRC 2 byte IBM [polynom 0xA001 = x16+x15+x2+1, use Modbus]: ( Cyclical Redundancy Check) - Special checksum, that is used for example in Modbus protocol. - CRC 2 byte IBM [polynom 0x8005 = x16+x15+x2+1, use DF1]: ( Cyclical Redundancy Check) - Special checksum, that is used for example in PmABradleyDF1 communication.
It is possible to enter where the checksum will be placed:
- to the end of the message (after ETX)
- before ETX
It is possible to enter which data of the message have to be included into the check sum:
- All data before the checksum
- All data before the checksum except the first character (except STX)
- All data before the checksum except the last character (except ETX)
- All data before the checksum except the first and the last character (except STX and ETX)
If the checksum is used, then it is automatically transmitted. On the other hand on each data receipt it is checked if the correct checksum is placed at the end. If it does not, the communication message is terminated with an error.
Checksum calculation can also be done by the PmBuffer.CalcCheckSum method. |
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| Enable character replacement in the received data | Can be enabled if we want to replace the really accepted character with another one in the received data.
It is used most often when we want to save the received data to the value of the String type but in the received data the character 00hex (binary zero) can appear. In this case it is necessary to replace the character 00hex by another one because the character 00hex is regarded as the end of the text. See the description further.
The character value is entered in decimal, see The ASCII table. |
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The communication description by means of the PmaCommMsg object
Message parameters:
| There is HexaString only in 'Data-sent' | If checked, then it is presumed that there is only a single variable of the String type on the "Data-sent" tab. The content of this variable is so-called HexaString that can be used in order to transfer binary values in the String type as follows:
For example if you are to send 4 bytes with binary values of 01 A0 00 B3 (in hexadecimal), then it is necessary to set into the variable of the String type the value of "01A000B3" - i.e. the text of 2*4=8 characters. This way you can easily transfer data containing binary zeroes, that are forbidden in the value of the String type.
The Pm.TransformValue(240) method can be used for preparation of the transferred data. |
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| There is HexaString only in 'Data-received' | The sane as the previous configurator, only for the "Data-received" tab.
The Pm.TransformValue(241) method can be used for processing the received data. |
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| This message will use next parameters (replace protocol parameters) | (only for messages of the Master type) If checked, then the same configurators are displayed as for protocol parameters (whether to use the STX, ETX characters, the checksum). These parameters overrule the protocol parameters during the message transfer. |
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Thus really received/transmitted data has for example the following format:
[STX] "user data" [ETX] [checksum]
Note: Fields inside the square brackets can be modified by the above mentioned parameters. The designer then configures only the sections marked as "user data" on the
Data-sent and
Data-received tabs.
The
PmaCommMsg object can be set for messages of the
Master or
Slave type (see the "
PmaCommMsg > Parameters > Message type" configurator). The transfer procedure for these message types is following:
-
Master type:
- The object transmission is started either automatically (according to the "
AutoRun transfer enabled" configurator) or by the
Run method.
- The object is placed into the inner queue of waiting
Master objects (more
PmaCommMsg objects may be waiting for data transfer simultaneously).
- When the object get its turn, the
onBeginOfTransfer event is triggered. The designer can then set the data for example on the "
Data-sent" tab (but not necessarily if already set).
- The data on the "
Data-sent" tab page is processed (e.g. the conversion from
HexaString to binary form, the
STX,
ETX characters or checksum are added).
- The processed data is then sent via Ethernet or via the computer's serial port.
- If there are no variables on the "
Data-received" tab, then the transmission is terminated (the
onEndOfTransfer event is triggered). This is used if the other side does not send any response.
- If there are variables defined on the "
Data-received" tab, then the object waits for message receive from the other side.
- After the message is received, the received data is checked and processed (the
STX,
ETX, and checksum is verified. If set, then the conversion into
HexaString is done), the processed data is then saved into the "
Data-received" tab and the
onDataReceive event is triggered.
-
Slave type:
- The object waits for receiving the data from other side. When the Master messages are being transferred the Slave message receive is blocked, i.e. Master messages are preferred.
- After the message is received from the other side it is decided which
Slave object this message belongs to. If there is only a single object of the
Slave type, then the received data is transferred there, otherwise the procedure goes as follows:
The received message length is compared to requested messages (defined on the "
Data-received" tab). The received message is assigned to the object that requested message of equal length (as the received message). If there are two objects requesting the same message length, then it is assigned to the first object - this object is then moved to the end and the next message (of the same length) is assigned to the second object, etc.
- After the received data are saved into the "
Data-received" tab of corresponding object, the
onDataReceive event is triggered. The designer can then set the data for sending on the "
Data-sent" tab.
- If there are no variables on the "
Data-sent" tab, then the transmission is terminated (the
onEndOfTransfer event is triggered). This is used if the PRMOTIC is not requested to send a response to the other side.
- If there are variables defined on the "
Data-sent" tab, then the object modifies the data from this tab (e.g. the conversion from
HexaString to binary form, the
STX,
ETX characters or checksum is added) and the processed data is then sent to the other side.
:
- If you place an item of the
String type into the "
Data-sent" tab, then the message length can me
modified dynamically (text of variable length can be assigned to the value of the
String type).
- If not set, then the text is set without conversion. This is handy for text oriented communications.
In order to create the formatted text easily, the
PmFormat object can be used (e.g. in the
onBeginOfTransfer event).
- If you place an item of the
String type into the "
Data-received" tab, then it is possible to
receive messages of variable length.
- If not set, then the received text is saved without conversion. This is handy for text oriented communications. In order to process the text easily, the
Pm.StringScan method can be used (e.g. in the
onEndOfTransfer event).
For the item of the
String type on the "
Data-received" tab, it is necessary to set the length (in order to let the driver know what length is expected to be received). This can be done:
- either directly on the tab by filling of the "
Length" configurator
- or it is possible to leave this field blank and then to set the string of required length into the item of the
String type by the
PmaCommMsg.ReadVars method before the transmission of the message.
If the item of the
String type is the last item on the tab, then the variable number of characters is received up to the text string length set by the above mentioned method.
If the item of the
String type isn't the last item on the tab, then the number of characters that corresponds exactly to the string length set by the above mentioned method, is received into this item.
:
The common problem is receiving the data including the
binary value 0 (further
00hex). Direct receiving of such data to the value of the
String type does not work well because the
String is always interpreted as a text terminated by the
00hex character. The data are received, but only the characters before the
00hex value are accessible. There are the following ways to solve this problem:
- or do not use variables of the
String type on the tabs
Data-sent or
Data-received, but use directly the
Byte,
Integer, etc. variables instead. This way disables receiving and sending the data of dynamically fluctuating length.
- or to use the "
Enable character replacement in the received data" configurator and to set there that the received character
00hex is to be replaced for example by the
"#" character - if you know that the
"#" character is not used in the transferred data (the
"#" character has the value
35, see
The ASCII table). Then you can receive data only into a single data item of the
String type and text there will sometimes contain the
"#" character (instead of
00hex). Such text can then be processed (e.g. in the
onEndOfTransfer event), for example, by the
Split VBScript function that separates the text according to the separator
"#".