Visca protocol

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VISCA is a standard developed by Sony for communication with video cameras . It describes both a protocol and an (optional) network topology . VISCA can automatically address devices without having to set anything in the device itself. Up to seven devices can be addressed via a single serial interface ( RS-232 or RS-485 ).

Interface and wiring

VISCA can basically be implemented with RS232, RS485 and other serial interfaces. The wiring provides that the devices are connected in a circle. This means that the transmission line of the first device goes to the reception line of the second device, the transmission line of the second to the reception line of the third and so on. The transmission line of the last device then goes back to the receiving line of the first device. This creates a closed circle similar to a token ring network.

The data packets may have to pass through all devices. Only the addressed device uses the data packet intended for itself and then sends on receipt and execution confirmations, which are sent back to the sender at the end of the chain.

The sender can for example recognize whether a command in the chain has remained unprocessed when he receives the packet back.

The standard communication parameters for RS232 or RS485 communication with VISCA are 9600 baud , 8  data bits , 1  start bit , 1  stop bit , no parity bit . Some cameras can also be addressed with higher baud rates. The detection usually takes place automatically.

protocol

The VISCA protocol has a relatively simple structure. It always consists of 3 to a maximum of 16 bytes . The first and the last byte have a fixed meaning. The remaining bytes are free. The last byte is always the end character FFh. Otherwise it must not appear within the VISCA package!

VISCA packages can be divided into commands and responses. Commands are sent from the controller (e.g. PC or control panel) to the cameras. Answers are the confirmation telegrams from the cameras.

VISCA commands always begin with the address of the addressed device (81h to 87h). If the address is 88h, all devices are addressed. 88h is the so-called "broadcast address".

VISCA replies always begin with the reply address of the responding device (90h to F0h). The reply address is different from the address of the addressed device in the command. This allows you to see directly whether it is a command or a response. So has z. B. the first device in the chain has the address 81h and its reply address is 90h. The second device has the address 82h and its reply address is A0h. The third device has the address 83h and its reply address is B0h and so on.

Example:

The controller asks the first camera for the current zoom value . It sends the command 81 09 04 47 FF (hex). The camera responds, for example, with 90 50 00 00 00 00 FF (hex). The 90h is the reply address of the camera with the address 81h.

addressing

Addressing takes place automatically with VISCA. Addresses are assigned in the order in which the devices are wired. The controller sends the packet 88 30 01 FF (hex). The first device in the chain analyzes the packet and is assigned the address 81h, which is derived from the third byte in the packet (01h). It increments the byte by one (02h) and sends the packet, which is then 88 30 02 FF, to the second camera. The second device in the chain is assigned the address 82h and so on. The last device in the chain sends the packet back to the controller. Based on the response from the chain, the operator can see how many cameras are in the chain. Up to seven cameras are allowed.

Differences to the Pelco D / P protocol

The main difference to the Pelco D / P protocol is the lack of a checksum in VISCA and the type of addressing. With Pelco-D / P the address is permanently set or programmed in the device (via DIP switches or similar), with VISCA it is assigned dynamically and results from the wiring sequence. Therefore, with VISCA a greater effort has to be made with the wiring. VISCA is also more susceptible to line breaks (affects the entire chain) while Pelco-D / P is more robust due to the star-shaped structure.

Therefore, VISCA is suitable for use over short distances and rather not for a larger area.

Pelco-D / P allows addressing of up to 256 different cameras while VISCA only knows up to seven addresses.

There are also protocol converters from Pelco-D / P to VISCA to bring both systems together and to combine the advantages and disadvantages.

distribution

VISCA is in almost all camera blocks from SONY such as B. FCB-EX480, FCB-EX780 etc. available. These camera blocks are mainly used in the security sector and are also integrated in products from other manufacturers. There are also cameras with integrated pan-tilt heads from SONY such as B. EVI-D31 or EVI-D100. Since these cameras are not suitable for use outdoors or in an industrial environment, there are also control units for robust, weatherproof pan-tilt heads that support the VISCA protocol and SONY cameras such as B. DCP-30 or DCP-18 from GNT.

Web links

Individual evidence

  1. ↑ libVISCA Home Page , libVISCA, accessed on 30 March 2014
  2. ↑ http://www.gnt.biz/dcp18n.htm