Beijer Electronics SCOM Protocol EN - Manual (Page 17)

Protocol Description and Timings
Elektronik-Systeme Lauer, MAEN976 17
Conditions for Response Message
The conditions for a response message generated by the monitor are described below:
Delayed Response
If a monitor receives a SCOM message with a broadcast address from the RS485 in-
terface and the command is completed, the generated response message is sent after
a delay time. This is required, because many monitors can be attached to the RS485
bus, but only one monitor can claim the line for its communication. The delay guar-
antees that each response message is sent one after one, even if all monitors have re-
ceived the broadcast message and processed it at the same time, avoiding a collision
on the RS485 bus.
The length of the delay time depends on the monitor address and maximum length
of a response message, and is calculated according to the following formula:
Interface Address condition Command status Response
RS232 Address does not match
monitor address not EXECUTED No response
not EXECUTED No response
Address matches monitor
address if COMPLETED ACK
if FAILED NAK
Address is Broadcast
address (0xFF) if COMPLETED ACK
if FAILED NAK
RS485 Address does not match
monitor address not EXECUTED No response
not EXECUTED No response
Address matches monitor
address if COMPLETED ACK
if FAILED NAK
Address is Broadcast
address (0xFF) if COMPLETED Delayed ACK
if FAILED No response
Ethernet Address does not match
monitor address not EXECUTED No response
not EXECUTED No response
Address matches monitor
address if COMPLETED ACK
if FAILED NAK
Address is Broadcast
address (0xFF) if COMPLETED No response
if FAILED No response
(2.5 + Lr) x 10 x N
Te =
b
Te: Broadcast command response [s]
Lr: Maximum length of response message
N: Monitor address + 1
b: Baud rate [bits/second]