Comtech EF Data OMS11 User Manual
Page 66
OMS11 1:1 Redundancy Switch
Appendix B
TM133 – Rev. 1.1
B-
3
02h + FOh + 2Ah + 09h + 03h + DFh + FEh = 305h.
Since the only concern is the modulo 256 (modulo 1 00h) equivalent (values that can be
represented by a single 8-bit byte), the checksum is 05h.
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 =
]
B.0.3 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-SYN-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 DMD15) 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-SYN 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. These serve 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 (such as increasing or decreasing the transmit
power level of a DMD15 modulator), the FSN is incremented after each message packet. When
the FSN value reaches 255, it overflows and begins again at zero. The FSN tag is a powerful tool
that assures sequential information framing, and is especially useful where commands require
more than one message packet.
The full handshake/acknowledgment involves a reversal of source and destination ID codes in the
next message frame, followed by a response code in the
packet from the equipment under control.