Making a Camera PTZ Decoder

Making a Camera PTZ Decoder

by Michael Shaw, July 2012

 This article explains the design of a Pan-Tilt-Zoom (PTZ) decoder that is used with a camera surveillance system. These systems are used for remotely positioning surveillance cameras and uses twisted pair RS485 for the physical connection. I made this decoder for use with a Maxwell MP-101 electronic power panner, and since this unit only has pan and tilt capability, this article is really about making a pan/ tilt decoder.

PTZ Decoder Interfaced with the Maxwell MP-101 Power Panner

The serial communications format that’s used to talk to the PTZ decoder has been standardized over the years, with the most popular format being the Pelco “D” protocol. I got the specifics of the Pelco D protocol from the website at CommFront,  http://www.commfront.com/RS232_Examples/CCTV/Pelco_D_Pelco_P_Examples_Tutorial.HTM. The picture in figure1 shows a DVR 4 camera system with the PTZ control in the bottom right panel. The DVR system controls a particular PTZ positioning device using a specific address for each decoder. Up to 255 different addresses can be assigned.   

Figure 1, example of a DVR Pan Tilt control panel

The Pelco “D” serial protocol will work with baud rates from 300 baud  up to 9600 baud. The protocol consists of a 7 byte control message that contains the information needed for controlling a PTZ device. The information in this control message contains the address of the device being controlled and the direction and speed of motion. The format of the command is as follows: (note: all values in hex)

Byte 1	Byte 2	Byte 3	Byte 4	Byte 5	Byte 6	Byte 7
Sync	Camera Address	Command 1	Command 2	Data 1	Data 2	Checksum

Byte 1 (sync), the synchronization byte, fixed at FF

Byte 2 (Camera Address), defined range is between 1-FF

Byte 3 (Command 1), first set of control parameters (explained below)

Byte 4 (Command 2), second set of control parameters (explained below)

Byte 5 (Data 1), pan speed parameters ranging from 00 (stop) to 3F (high speed) and FF for turbo (maximum speed of device)

Byte 6 (Data 2), tilt speed parameters ranging from 00 (stop) to 3F (maximum)

Byte 7 (Checksum), sum of bytes (excluding sync byte) then modulo 100

Bytes 3 and 4 are used for the Command 1 and Command 2 control information and are encoded in the following format:

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
Command 1	Sense	Reserved	Reserved	Auto/Manual Scan	Camera On/Off	Iris Close	Iris Open	Focus Near
Command 2	Focus Far	Zoom Wide	Zoom Tele	Tilt Down	Tilt Up	Pan Left	Pan Right	Fixed to 0

The Maxwell MP-101 electronic power panner has a connector that is used by a remote control to control the position of the camera. This connector will be used for interfacing the decoder to the Maxwell power panner. All the Maxwell power panner needs is a contact closure to activate the desired pan or tilt motion. The decoder uses relays to provide this contact closure, so all that’s needed is to detect the Tilt up/down and Pan left/right commands in the Pelco D message.

I used a PICAXE 18M2 chip as the controller in this decoder design. The PICAXE 18M2 chip is real easy to work with and provides enough I/O pins to do what I want to do. I’ll need 4 outputs for the relays that will be controlling the direction of motion; these are tilt up, tilt down, pan left and pan right. I also want to be able to provide a way for assigning an address to the decoder, so I’ll need 8 pins for that.  To make it easy, I’ve fixed the baud rate at 4800, but I’ll need a pin for the serial input coming from the RS485 transceiver chip and an output pin for a status LED. The schematic for the decoder is pictured in figure 2.

Figure 2, PTZ Decoder Schematic

The software for the decoder is pretty straight forward. When it first starts, it reads the unit address switches then waits for a serial message to arrive. When a serial message is received, it verifies that the checksum is correct and then checks the unit address for a match. If the addresses match then it checks for the pan and tilt commands in the command 2 message.

The software checks for a total of 8 conditions for the pan tilt operations since pan and tilt control could be combined in a single message. The following is a truth table and the resulting values for the various pan/tilt conditions:

Command 2	Tilt Down Bit 4	Tilt Up  Bit 3	Pan Left Bit 2	Pan Right Bit 1	Fixed to 0 Bit 0
	16	8	4	2	1
2				X
4			X
8		X
16	X
10		X		X
18	X			X
12		X	X
20	X		X

 

The design prototype was constructed in about a day on a PC board from Radio Shack. The components were purchased from several different suppliers. The average cost to build the decoder was around $50.

Picture of the PTZ Decoder Prototype

The Decoder Parts List

Ref	Quantity	Description	Part Number	Vendor
RLY1-RLY4	4	SPDT 5V Relay	275-0240	RadioShack
PCB1	1	Prototype PC Board	276-168	RadioShack
U2	1	RS485 transceiver, 5V, 8pin DIP	LTC485CN8#PBF-ND	Digi-Key
U3	1	5V voltage regulator	COM-00107	SparkFun
U1	1	PICAXE 18M2 Microcontroller	COM-10187	SparkFun
SOCKET1	1	18 pin DIP Socket	PRT-07940	SparkFun
SOCKET2	1	8 pin DIP Socket	PRT-07937	SparkFun
J1	1	DC Power Jack	PRT-00119	SparkFun
J2	1	2-pin PCB mount Spring Terminal	PRT-08073	SparkFun
C2	1	Capacitor, 100uf, 50VDC	272-1044	RadioShack
C3	1	Capacitor, 10uf, 10VDC	55047371	RadioShack
C1,C4	2	Capacitor, .1uf, 50VDC	399-4266-ND	Digi-Key
R5	1	330 ohm 0.25W resistor	271-1315	RadioShack
D5	1	Green LED	COM-09592	SparkFun
RN1	1	10K ohm, 9 resistor network	CSC10KY-ND	Digi-Key
R1-R4	4	10K ohm, 0.25W resistors	271-1335	RadioShack
SW1-SW8	1	8 position DIP switch	COM-08034	SparkFun
Q1-Q4	4	2N3904 NPN Small Signal Transistors	COM-00521	SparkFun
D1-D4	4	1N4005 Diodes	276-1104	RadioShack
PWR1	1	Wall Adapter Power Supply, 9VDC	TOL-00298	SparkFun
	AR	24 AWG hookup Wire

This decoder design works as expected with a typical DVR setup and is relatively inexpensive to make. You no longer have to be satisfied with a single camera view and can position the camera remotely to view the area of interest. In the next design I will incorporate PWM controlled motors into the circuit, and eliminate the relays. This will allow me to create a complete PTZ unit.

Below is a copy of the PICAXE code

 'PTZ Decoder FOR PICAXE 18M2
 'decodes Pelco "D" type signals
 'By Mike Shaw 7/2012
 'Settings Baud 4800,8,N,1
 Symbol LED    = C.2
Symbol Relay1 = C.6     ' Tilt Up
Symbol Relay2 = C.7     ' Tilt Down
Symbol Relay3 = C.0     ' Pan Right
Symbol Relay4 = C.1     ' Pan Left
Symbol stx   =      b0
Symbol adr   =     b1
Symbol cmd1  =   b2
Symbol cmd2  =   b3
Symbol dat1  =     b4
Symbol dat2  =     b5
Symbol chks  =     b6
Symbol my_adr = b7
Symbol tmp   =     b8

Symbol RX_PIN = C.5                 'Input from RS485 chip
Symbol RX_BAUD = T4800       'Baud fixed at true 4800
Gosub GetADD
Do
 Gosub GetPacket
 If adr = my_adr Then
 Gosub HandlePacket
 End If
 Loop
 GetPacket:
 Do
 SerIn RX_PIN,RX_BAUD,stx,adr,cmd1,cmd2,dat1,dat2,chks
 tmp = adr + cmd1 + cmd2 + dat1 + dat2
 Loop Until tmp = chks
 Return

 GetADD:  ' Read then Display Unit Address
 my_adr = pinsB ^ 001111     'invert address
 for b0 = 1 to my_adr
 high LED
 pause 300 
 low LED
 pause 300
 next b0
 high LED
 Return

 HandlePacket:
 low LED
 if cmd2 = 2 then high Relay3 End If     'Pan Right
 if cmd2 = 4 then high Relay4 End If     'Pan Left
 if cmd2 = 8 then high Relay1 End If      'Tilt Up
 if cmd2 = 10 then high Relay3 End If    'Relay1 Pan Right & Tilt Up
 if cmd2 = 12 then high Relay4 End If    'Relay1 Pan Left & Tilt Up
 if cmd2 = 16 then high Relay2 End If'   'Tilt Down
 if cmd2 = 18 then high Relay3 End If    'Relay2 Pan Right & Tilt Down
 if cmd2 = 20 then high Relay4 End If    'Relay2 Pan Left & Tilt Down
 pause 1000   ' 1 sec delay
 low Relay1,Relay2,Relay3,Relay4
 high LED
 Return

ORDER IT HERE!