Table of Contents
Advertisement
RCS20 M:N Redundancy Switch
Revision 15
Remote Operations
MN-RCS20 and CD-RCS20
For a decimal checksum calculation, the equivalent values for each information field are:
0 + 2 + 240 + 42 + 9 + 3 + 223 + 254 = 773;
773/256 = 3 with a remainder of 5.
This remainder is the checksum for the frame.
5 (decimal) = 05h = 0101b = <CHECKSUM>
A.1.3 Frame Description and Bus Handshaking
In a Monitor and Control environment, every message frame on a control bus port executes as a packet
in a loop beginning with a wait-for-SYNC-character mode. The remaining message format header
information is then loaded, either by the M&C computer or by a subordinate piece of equipment (such as
the RCS20) requesting access to the bus. Data is processed in accordance with the OPCODE, and the
checksum for the frame is calculated. If the anticipated checksum does not match then a checksum error
response is returned to the message frame originator.
The entire message frame is discarded and the wait-for-SYNC mode goes back into effect.
If the OPCODE resides within a command message, it defines the class of action that denotes an
instruction, which is specific to the device type, and is a prefix to the DATA field if data is required. If the
OPCODE resides within a query message packet, then it defines the query code, and can serve as a
prefix to query code DATA.
The Frame Sequence Number (FSN) is included in every message packet, and increments sequentially.
When the M&C computer or bus-linked equipment initiates a message, it assigns the FSN as a tag for
error control and handshaking. A different FSN is produced for each new message from the FSN
originator to a specific device on the control bus. If a command packet is sent and not received at its
intended destination, then an appropriate response message is not received by the packet originator. The
original command packet is then re-transmitted with the same FSN. If the repeated message is received
correctly at this point, it is considered a new message and is executed and acknowledged as such.
If the command packet is received at its intended destination but the response message
(acknowledgment) is lost, then the message originator (usually the M&C computer) re-transmits the
original command packet with the same FSN. The destination device detects the same FSN and
recognizes that the message is a duplicate, so the associated commands within the packet are not
executed a second time. However, the response packet is again sent back to the source, as an
acknowledgment in order to preclude undesired multiple executions of the same command.
To reiterate, valid equipment responses to a message require the FSN tag in the command packet; this
serves as part of the handshake/acknowledge routine. If a valid response message is absent, then the
command is re-transmitted with the same FSN. For a repeat of the same command involving iterative
processes, the FSN is incremented after each message packet. When the FSN value reaches 255, it
A–4
Advertisement
Table of Contents
