Operating instructions

DVB3030/DVB3030L Digital Video Broadcast Modulator User Interfaces
TM052 - Rev. 3.5 4-23
5 (decimal) = 05h = 0101b = <CHECKSUM>
4.3.3 Flow Control and Task Processing
The original packet sender (the M&C computer) relies on accurate timeout information with
regard to each piece of equipment under its control. This provides for efficient bus
communication without unnecessary handshake overhead timing. One critical value is
designated the Inter-Frame Space (FS). The Inter-Frame Space provides a period of time in
which the packet receiver and medium (control bus and M&C computer interface) fully recover
from the packet transmission/reception process and the receiver is ready to accept a new
message. The programmed value of the Inter-Frame Space should be greater than the sum of
the "turnaround time" and the round-trip (sender/receiver/bus) propagation time, including
handshake overhead. The term "turnaround time" refers to the amount of time required for a
receiver to be re-enabled and ready to receive a packet after having just received a packet. In
flow control programming, the Inter-Frame Space may be determined empirically in accord with
the system configuration, or calculated based on established maximum equipment task
processing times.
Each piece of supported equipment on the control bus executes a Radyne Link Level Task
(RLLT) in accordance with its internal hardware and fixed program structure. In a flow control
example, the RLLT issues an internal "message in" system call to invoke an I/0 wait condition
that persists until the task receives a command from the M & C computer. The RLLT has the
option of setting a timeout on the incoming message. Thus, if the equipment does not receive
an information/command packet within a given time period, the associated RLLT exits the I/0
wait state and takes appropriate action.
Radyne equipment is logically linked to the control bus via an Internal Input/Output Processing
Task (IOPT) to handle frame sequencing, error checking, and handshaking. The IOPT is
essentially a link between the equipment RLLT and the control bus. Each time the M&C
computer sends a message packet, the IOPT receives the message and performs error
checking. If errors are absent, the IOPT passes the message to the equipment's RLLT. If the
IOPT detects errors, it appends error messages to the packet. Whenever an error occurs, the
IOPT notes it and discards the message; but it keeps track of the incoming packet. Once the
packet is complete, the IOPT conveys the appropriate message to the RLLT and invokes an I/0
wait state (wait for next <SYN> character).
If the RLLT receives the packetized message from the sender before it times out, it checks for
any error messages appended by the IOPT. In the absence of errors, the RLLT processes the
received command sent via the transmitted packet and issues a "message out" system call to
ultimately acknowledge the received packet. This call generates the response packet conveyed
to the sender. If the IOPT sensed errors in the received packet and an RLLT timeout has not
occurred, the RLLT causes the equipment to issue the appropriate error message(s) in the
pending equipment response frame.
To maintain frame synchronization, the IOPT keeps track of error-laden packets and packets
intended for other equipment for the duration of each received packet. Once the packet is
complete, the IOPT invokes an I/0 wait state and searches for the next <SYN> character.
4.3.4 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 DVB3030) requesting access to the bus. Data is processed in