Lumel RE15, Руководство пользователя, Страница: 15 / 28

15
4. ERROR CODES
When the master device is broadcasting a demand to the slave
device then, except for messages in the broadcast mode, it expects
a correct answer. After sending the demand of the master unit, one
of the four possibilities can occur:
v
If the slave unit receives the demand without a transmission
error and can execute it correctly, then it returns a correct
answer,
v
If the slave unit does not receive the demand, no answer is
returned. Timeout conditions for the demand will be fulfilled in
the master device program.
v
If the slave unit receives the demand, but with transmission
errors (even parity error of checking sum LRC or CRC), no
answer is returned. Timeout condition for the demand will be
fulfilled in the master device program.
v
If the slave unit receives the demand without a transmission
error but cannot execute it correctly (e.g. if the demand is, the
reading-out of a non-existent bit output or register), then it
returns the answer including the error code, informing the
master device about the error reason.
v
A message with an incorrect answer includes two fields
distinguishing it from the correct answer.
The function code field:
In the correct answer, the slave unit retransmits the function code
from the demand message in the field of the answer function code.
All function codes have the most-significant bit (MSB) equal zero
(code values are under 80h). In the incorrect answer, the slave unit
sets up the MSB bit of the function code at 1. This causes that the
function code value in the incorrect answer is exactly of 80h greater
than it would be in a correct answer.
On the base of the function code with a set up MSB bit the program
of the master device can recognize an incorrect answer and can
check the error code on the data field.