Dnet1chconv revision 0 Single-Channel DeviceNet rs-232 Serial Gateway User’s Manual sst, a division of Woodhead Canada Limited

Setting Up the Serial Link

1. Reception Overview

The DNET1CHCONV receives characters into its internal character queue. The DNET1CHCONV then takes the first character out of the queue and places it into the back receive buffer. If the character that was placed into the back buffer is the Receive End Delimiter or the back buffer contains the Max Receive Length of characters, The DNET1CHCONV will copy the back buffer into the Receive Buffer and change the Receive Transaction Identifier (RXID) to notify the PLC that new data is ready.

Section 5.4.2 describes the Receive Swap settings

Section 5.3.4 describes the Receive End delimiter Settings

Appendix A describes the receive formatting object in more detail.

It is important to note that the IO assembly object does not process the content of the data given it. It only disassembles the received data and assembles the outgoing data to/from other logical sources in the device. Due to the nature of DeviceNet messaging, a field changed by an explicit message that is also set by an IO message will change momentarily to the explicit value and then immediately change back to the IO data. This will cause erroneous operation of the device. In short, if you wish to use explicit messaging for I/O, do not allocate an I/O connection.

2. Transmission Overview

The DNET1CHCONV will transmit the current data in the Transmit Buffer whenever the Transmit Transaction Identifier (TXID) is changed. These fields may be changed explicitly and are also available in the IO messages. The information that is stored in the Transmit Buffer is first parsed by the DNET1CHCONV, which changes the order of data based on the setting of the Transmit Byte Swap and the amount of data sent based on the setting of the Transmit End Delimiter. The Transmit Byte swap option is used to align the data to your PLC’s data table. The maximum amount of information that will be received at one time is determined by the Transmit Character Buffer Length (in the device net set up).

Section 5.4.2 describes the Transmit Swap settings

Section 5.3.5 describes the Transmit End delimiter Settings

Appendix A describes the transmit formatting object in more detail.

It is important to note that the IO assembly object does not process the content of the data given it. It only disassembles the received data and assembles the outgoing data to/from other logical sources in the device. Due to the nature of DeviceNet messaging, a field changed by an explicit message that is also set by an IO message will change momentarily to the explicit value and then immediately change back to the IO data. This will cause erroneous operation of the device. In short, if you wish to use explicit messaging, do not allocate an IO connection.

If you set the Tx Buffer Length to 0 and the DNET1CHCONV receives more than the number of bytes defined in Max TX Buffer Length in a consumed DeviceNet message from the Master, the DNET1CHCONV will drop the I/O connection or return an error of “too much data”.

3. Physical Layer Properties

4. Hardware Handshaking

5. The Receive Delimiter

Please note that the Receive Delimiter operation is always active, unlike the Transmit Delimiter operation (described below), which can be enabled or disabled. If you want to receive a constant string equal to the Max Receive Length, and do not want the delimiter operation to initiate a partial produce on DeviceNet, then make sure your serial device does not send the designated Rx Delimiter Character.

6. The Transmit Delimiter

The Transmit Delimiter operation is keyed off the Transmit Buffer Length byte in the DeviceNet data that the DNET1CHCONV consumes from the Master. This Transmit Buffer Length byte is the second byte in the consume assembly. See Table 6 -13.

The Transmit Delimiter will be used if the Transmit Buffer Length equals 0. If the Tx Buffer Length is a non-zero value, the DNET1CHCONV will ignore the transmit delimiter and transmit the specified number of bytes.

The transmit delimiter can be set to any valid I/O character that can be received over the link. Be very careful not to set the delimiter to a value outside of the valid range for your data bits (Note: A data bit size setting of 7 will only allow you a delimiter range of 0-127 dec., 00-7Fhex). If you do not have the specified delimiter in the string you send to the DNET1CHCONV and the TX Length is not 0, the DNET1CHCONV will transmit the entire received message. If your PLC or PC Master device does not support variable length IO messages, this will have the effect of putting garbage characters at the end of your string.

7. Status Byte Description

Table 5‑12 Serial Status Byte

• 
  TX Buffer Overflow
  

• 
  Rx Buffer Overflow
  

• 
  Receive error
  

• 
  Data in Tx Buffer
  

• 
  Data in Rx Buffer
  

6. Assembly Object Formats

The produce (receive) and consume (transmit) assembly object formats vary according to 2 specific parameter options: the Data Type and the Handshaking / Immediate Mode operation.

In this discussion “consume” and “produce” refer to data received from and sent out to the DeviceNet network by the DNET1CHCONV.

For this mode to work correctly in many PLCs with a scanner, the outgoing poll message must be constructed in a buffer, and when complete, should then be copied whole to the scanner’s file.

1. Assemblies

1. Theory of Operation

   1. The Transmit Record Algorithm

      1. Basic Theory of operation

The data flow and algorithm are shown below for reference.

Figure A‑5 Transmit Record Algorithm Functional Flowchart

As diagramed above, the serial data is set and then parsed if the serial data length is set to zero. The parser then sets the length on the serial data. If Record Protection is disabled, the data is immediately added to the serial port’s transmit queue. Also, whenever the Transaction Id is changed, the serial transmit data is sent to the serial port object.

The parser determines the length of the string by searching the string for a delimiter character. The algorithm can be configured to ignore, include or exclude the character.

2. The Receive Record Algorithm

1. Basic Theory of operation

The Receive Record Algorithm creates a DeviceNet object and presents the data in an ordered fashion to the DeviceNet I/O connection. The Receive Record object may be reached over DeviceNet at Class 0x72. If you are going to be using explicit messaging to communicate with the device, we recommend that you access the data from this point, and not the parameter object, because the parameter object obeys slightly different rules in order to ease the interface with today’s most common configuration tools.

The Receive Record Algorithm is diagramed above. The serial stream originates from the serial port object (0x70). It is looked at a byte at a time by the event detection system. If the event detection system detects a beginning event (the beginning delimiter is received or disabled), it closes the first data switch, allowing the serial data to accumulate in the back buffer. Once an end event is received (the End delimiter is received, or the back buffer is full) the event detection system will close the data switch and set a new data status. When the new data status is set, if the device is set up in auto increment mode, the object will automatically increment the record number and clear the new data bit. In this configuration, the user will not be allowed to set the Receive Record Number.

The SHORT_STRING data type is the DeviceNet data type used in the DNET1CHCONV. This data type has a 1-byte length.

The DNET1CHCONV is supported by a variety of DeviceNet. Short poll responses save bandwidth on the DeviceNet network, but many scanners do not support this functionality and will not allow communication with a device that responds in this manner. The DNET1CHCONV supports this non-compliant behavior through pad mode. This is a function where a character is appended to the end of the serial data in order to fill out the poll response. The default for pad mode is OFF, and the default for the pad character is 0 (NULL).

Note: Turn pad mode ON if you receive errors indicating that the I/O data response is too short.

The DNET1CHCONV also supports byte-reordering to support multi-byte PLC files. The DeviceNet network data-ordering scheme is little ending, meaning that the low byte of a multi-byte messages transmitted first. This means that if you are using a data file that is using a 16 bit or larger word size and you map the connection’s data directly into this space, your data bytes will show up swapped. The DNET1CHCONV implements byte reordering (swapping) in order to accommodate these PLCs. The swapping mechanism re-orders the I/O data so that is appears in a human readable form in a file with a little ending byte order. To enable byte swapping, determine the number of bytes in your data size. Subtract 1 from this number, and set the Receive Record object’s byte swapping attribute to this value (also available through the parameter object). Now, you must set the length of your message size to a multiple of 1+this value, or the DNET1CHCONV will not be able to swap the bytes correctly. The data will now be ordered properly in your DNET1CHCONV.

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