Option "CAN-Receive-ONLY"

(only available for certain terminals with suitable CAN-transceiver)

Note: This document is a work in progress. The screenshots in this document may look different in the final version !

Usually, every node in a CAN network must be able to receive and transmit. Even a node which only receives CAN telegrams affects the network because it sends a so-called acknowledge bit for every received frame. This may cause problems, especially when you connect the terminal to a network with the wrong CAN bitrate, or other wrong bit timing parameters (for example, the "sampling point"). If the CAN transmitter is disabled, the risk of causing severe problems in the network is greatly reduced, it can only cause non-function to the terminal itself but not to the 'outside world'.

This document describes how the CAN TRANSMITTER (which is a part in a CAN TRANSCEIVER) can be disabled in certain terminals.

Contents

See also:



Scope

Turning the CAN Transmitter off is a rarely used function, and it is only possible with newer CAN transceiver chips which are installed in certain terminals since 2004.

At the time of this writing, CAN-TRANSMIT could only be turned off (by software) in the following terminals...

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How to turn off the CAN TRANSMITTER from the system menu

Follow the following steps to disable the CAN TRANSMITTER, here for the "MKT-View Plus". In other terminals with different keyboards the procedure will be similar. In later devices (MKT-View II,III,IV) there are individual menu items for each CAN bus interface.

  1. Press the key combination to activate the system menu (for MKT-View: press the 2nd and 3rd function key simultaneously)
  2. With the rotary button or cursor keys, navigate to "System Setup" (or "System Setup / UNLOCK")
  3. Press the rotary button or the enter key to jump into that submenu
  4. Navigate to the menu item "CAN-TxEnable=xxx"
    (if there is no such item, either the terminal or the firmware do not support passive CAN reception)
  5. Press the rotary button or the enter key to switch to edit mode (for the parameter "xxx")
  6. To disable or enable the CAN TRANSMITTER, modify the parameter according to the table below
  7. Press the rotary button (or enter) to finish editing
  8. Select the menu item "Save & Exit" (press button/enter) to save the changes in non-volatile memory

The numeric value in the menu item "CAN-TxEnable=xxx" is an 8-bit value with one bit for every CAN interface. Since at the current time there are only up to 2 CAN interfaces in a terminal, only the lower 2 bits have a function (this may change in future). The following table shows the effect of changing the value.

CAN-TxEnable= ...

CAN 1 CAN 2

000

RX only RX only

001

RX+TX RX only

002

RX only RX+TX

003

RX+TX TX+TX

By default, 'xxx' will be 255 (= erased EEPROM cell) which means all CAN-Interfaces are able to transmit (same effect as the value 003, but this would turn on ALL CAN-transmitters for up to 8 CAN interfaces).

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Effects of turning the CAN TRANSMITTER off

There are a few things to be aware of when turning the CAN-transmitter off. It may cause problems, even if the the terminal is used to receive only !

The reason is the CAN controller's behaviour when receiving CAN frames. The reception of a CAN frame is only complete, if the receiver can "see" the acknowledge bit on the CAN bus. If the CAN transmitter (in the CAN transceiver) is disabled, there will be no ACK bit on the bus, if no other CAN node sends it. The effect is, if there are only TWO nodes on the bus (a transmitter and the terminal in "RX-ONLY"-mode), the transmitter will not be able to transmit successfully (because he won't get an acknowledge), and the recevier will not be able to receive anything (because the receiver does not see the ACK bit which it would normally send to the bus as a single dominant bit).

Furthermore, a CAN Receiver should be able to send CAN-Error-frames causing the transmitter to repeat the transmission. If the receiver cannot send error frames, the transmitter will not repeat frames which may have been corrupted by electrical interference, frame collisions, etc.

So, when using the terminal with the CAN transmitter disabled, make sure that "someone out there" can acknowledge the CAN frames.

Other things to be aware of when using the option "CAN-Transmitter-Off" :

Note: When calling the terminal's firmware bootloader from the system menu, the CAN interface will be reprogrammed, which includes activating the CAN transmitter.

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File: ..?..\uptwin1\help\canrx_01.htm
Author: W.Büscher, MKT Systemtechnik
Last modified: 2014-08-18 (ISO8601, YYYY-MM-DD)

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