Grass Valley Switcher Products - Manual (Page 123)

Switcher Products — Protocols Manual 123
Transmission Media and Protocols
line event and respond to one sent by the switcher with an appropriate
return code message.
Transmission Media and Protocols
The switcher/DPM interface is designed as a point-to-point link. Messages
are delivered in an HDLC-like frame consisting of an 8-bit opening flag, a
variable-length data field, a 16-bit CRC, and an 8-bit closing flag. Within the
transmission frame, messages are delivered via the data field.
A 307.2K baud FM0 synchronous serial link with imbedded clock is used
for deterministic communications. Data rate and frame format are not
adjustable. HDLC is a synchronous protocol with built-in mechanisms for
error detection and packet detection.
Not all UARTs support this protocol. Those that do handle the packetiza-
tion, zero stuffing and CRC insertion/detection. A Motorola 68302 along
with a 68195 are used in the switcher for this interface. A Motorola 68360 or
Zilog 8030 or 8530 also support this protocol.
Unlike asynchronous protocols, communications is frame (packet) based.
A transmit underrun is not permitted in a synchronous protocol and will
abort the transmission of the frame and the receiver will discard it.
The physical connection between the switcher and the digital effect device
(DPM) is point to point with two RS-422 pairs, one for each direction.
The connector provided for this interface is a female DB-9. It is located on
the rear of the switcher frame and labeled “A1”. The pinout is shown
below.
Table 115. Return Code Message Data
Function Pin
Tx + (switcher to DPM) 3
Tx - 8
Rx + (DPM to switcher) 7
Rx - 2
Ground 1, 4, 6, 9