Grass Valley Switcher Products - Manual (Page 96)

96 Switcher Products — Protocols Manual
Section 3 — Peripheral Bus II Protocol
minimum. A Digital Disk Recorder should implement Play, Stop and Cue
at minimum, and use Recall commands to load clips.
Command Timing and Frame Accuracy
It is possible to use the Peripheral Bus II protocol for frame accurate control,
but this is not guaranteed under all conditions. PBus II uses a serial inter-
connect and the commands are long. It is possible to saturate the serial link
with commands especially at lower baud rates, making the link non-deter-
ministic. For example, a recall command is 11 characters. At 9600 baud this
supports only 16 characters per field, sufficient for only one command per
field. At 38.4 Kb, there is link bandwidth to support 64 characters per field,
enough to send three recall and three trigger commands.
It is the responsibility of the controlling device to output commands at an
appropriate time so the controlled devices can respond in sync with other
activities of the system. The controlling device should provide and specify
deterministic responses to triggers and recalls.
Protocol Description
The Peripheral Bus II protocol provides a means for orderly communica-
tion by a controlling device to up to 24 peripheral controlled devices on a
serial data bus.
Hardware Interface
Communications follow the RS-485 standard at 9.6, 19.2, 38.4 k, 56.7, 76.8
Kb, or 115.2 KBaud with even, odd, or no parity. RS-485 is a derivative of
RS-422 that specifies controlled devices must disable their transmit drivers
when not sending information. This is required because all controlled
devices share a common connection (
Figure 7 on page 94 ).
RS-422 may be used instead of RS-485 only if a single line is connected from
the controlling device's transmitter to all the controlled devices’ receivers
and the controlled devices' transmitter lines are completely disconnected.
This configuration prevents the controlling device from receiving informa-
tion from the controlled devices. This disrupts Read, Write, and Query
functions, but these commands are not sent by Kalypso or Zodiak systems
(of Grass Valley Group switchers, only the Model 200/250 uses these com-
mands).
All devices connected to the peripheral bus must be set to exactly the same
communications standard. The recommended settings are 38.K baud and
no parity.