Sshqgl[ $ 5hprwh $fwlydwlrq, Hardware specifications, Bus packet format – Grass Valley XSwitch Feb 01 2005 User Manual

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The PVTV XSWITCH has remote activation capabilities. The following information is a communication
guide to assist with use of this functionality. Connector and pinout information is also supplied. ParkerVi-
sion does not offer a remote activation device. This device must be supplied by the customer.

A standard bus communication consists of a command sent by an originating device to a receiving device
with the receiving device returning a reply. If the reply is not received within a designated time period, then
the originating device has the option to retry or cancel the command.

Hardware Specifications

The computer may listen, talk, or interrupt bus comms at any time. The active lines to the RS-232 port are
TXD, RXD, and ground, as shown in the Installation Manual. No hardware handshaking is used. The RS-
232 port signal levels are +12 and -12 volts.

Care must be taken to insure that the electrical connections are of the correct polarity and voltage, or dam-
age may result. Be sure to consult the particular device connector diagrams for correct signal pinouts and
polarities.

Bus Packet Format

ParkerVision communication packets consist of 10-bit words containing 1 start bit, 8 data bits, and 1 stop
bit. The words are transmitted one bit at a time (serially), least-significant bit first. The data rate is fixed at
9600 baud. Successive words in a packet must follow one another within 20 milliseconds or the packet is
declared invalid. There must be a minimum delay of 40 milliseconds between successive packets. Any
computer connected to the communication bus must adhere to these packet specifications. The bus
comms are half-duplex.

Command and reply packets are organized similarly, the only difference being that reply packets do not
have a Command/Control field. Packets are ordered as follows:

Note: When using the Basic Protocol, the two CRC bytes are not used.

•

DEST: Destination Address field (1 word). Range: 0-255 binary. This is the address of the device to
which packet is being sent. Address 255 is a Broadcast address. Broadcast packets are received by
all devices, but no reply is sent. (Default Address 0)

•

SRC: Source Address field (1 word). Range: 0-254 binary. This is the address of the device from
which the packet is being sent. (Default Address 255)

•

LEN: Length of data field (1 word). Range: 1-256 (0=255) binary. The packet data field includes
words following the length word up to, but not including, the CRC field.

•

C/C: Command/Control field (1 or more words). Command codes are ASCII numbers ranging from 0
to 255. Control codes are binary numbers ranging from 0 to 47.

•

DATAn: Data field (0 or more words). The data field can be empty, or contain either ASCII or binary
words. If the data is ASCII, then, in the case of a command packet, data is separated by a comma (,),
and in the case of a reply packet, data is separated by a colon (:). Binary data has no explicit separator.
(see particular device command list for further information).

•

CRCL: Lower half of cyclic redundancy check (1 word). The CRC computes using the CRC-CCITT
polynomial: x^16 + x^12 + x^5 + 1

•

CRCH: Higher half of cyclic redundancy check (1 word)

Error recovery is handled by the originating device. This device may choose to terminate or re-transmit the
command.

LEN+3

LEN+4

LEN+5

DEST

SRC

LEN

C/C

DATA1

…

DATAn

CRCL

CRCH