3300 ACM Installation and Operation Manual Power Measurement Ltd.
G-4 Appendix G: Serial Communications Protocol
2.4 NETWORK TIMING
CONSIDERATIONS
Network timing for the transfer of packets between units on
the RS-485 loop must conform to the following rules.
1) The time between the end of a MASTER STATION
message request packet and the beginning of a SLAVE
STATION message response packet must not be less than
5 milliseconds.
T response min = 5 milliseconds
This is to provide the MASTER STATION with enough
time to prepare for reception of the message response
packet from the SLAVE STATION.
2) The time between the end of a MASTER
STATION message request packet and the beginning of a
SLAVE STATION message response packet must not
exceed 500 milli-seconds.
T response max = 500 milliseconds
Note that this is typically 100 milliseconds
for the 3300 ACM.
3) The minimum time between the end of any MASTER
STATION message packet and the beginning of the next
MASTER STATION packet is device dependent.
T master min = [device dependent]
This is equal to 100 milliseconds for the
3300 ACM.
4) The minimum time between the end of a SLAVE STA-
TION response packet and the beginning of the next
MASTER STATION message packet is device dependent.
T slave min = [device dependent]
Note that this is equal to 100 milliseconds for the 3300
ACM.
5) The maximum time between any two data bytes within a
packet must not exceed 50 milliseconds.
T byte max = 50 milliseconds.
Note that this is typically less than 1 millisecond for the
3300 ACM.
6) It is recommended that all Master station packet trans-
missions be prefixed with two null bytes to ensure that
the RS-485 data bus is stable before the sync byte is
transmitted.
2.3 BROADCAST PACKETS
Provisions have been made for the use of broadcast com-
mands within the RS-485 data transfer protocol. The purpose
of this is to allow all Slave devices to receive the same com-
mand from the Master station. This feature is very useful in
situations such as initial setup where all 3300 ACM meters
have the same setup parameters.
When broadcast packets are transmitted by the Master loop
controller, all Slave devices will receive and perform the packet
command but will not send a response packet. This is to
avoid the possibility of having more than one Slave Device
respond at one time. The Master Station must ensure that
commands sent via the broadcast mode do not attempt to
invoke a response from the Slave Devices.
To send broadcast commands to all 3300 ACM units on an
RS-485 loop, the Device Type Sub-field must be 253 decimal or
FD hex, and the destination unit address of the ADDRESS
INFORMATION field must be set to 0000 Hex.
Only when the destination address is set to zero will broad-
cast command packets be performed by the receiving 3300
ACM unit.
Courtesy of NationalSwitchgear.com
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