zondag 29 januari 2017

Program the Racal Cougar - Part 2


This is a follow up on my first post about the Racal Cougar 4 meter surplus radio and how to program this set using an Arduino. This part is about communication over the F line and the protocol used. 

A serious warning about hooking something up to your Racal Cougar an try to program it: do so at your own risk !

Communication


Besides connecting the microphone or MA 4730A Remote Control Unit the 7 pin connector is also used to connect the set to a device such as a programmer or fill gun.

Pinout of Audio connector

This connector has the following functions available:


Pin
Description
A
Microphone input
B
Microphone return
C
10V in or output voltage, limited to 100 mA
D
Audio out
E
Earth / 0V
F
PTT  or 4k bit / sec serial data in and out
G
Squelch output

Some pins have multiple functions available which are not displayed in above table, for a complete description of the table see the technical manual available at the Crypto Museum website

For sending and receiving commands to the set our pin of interest is the F pin. This pin has 2 functions:

PTT
Sets the transceiver in transmit mode and uses the AF signal (1mV RMS) from the microphone input (A pin) to modulate the signal.

Serial data in / out
Sets the transceiver in data mode, this mode allows an external device to communicate with the internal processor which controls the device. This communication should not be confused with packet like data where the set communicates data over the ether.
 

Both enabling the PTT and serial data functionality is done by putting the F pin into a low state. The difference between both modes is the duration pin low state. When the pin is kept low for 1 mS or more the ‘signal’ will be interpreted as a PTT signal otherwise it  is recognized as a data signal and interrupts the processor to switch to data mode.

Serial data

Data is send over the F line using PWM (Pulse Width Modulation). This line its normal state is high at about 8 volt and data is send by pulling it low for periods of time. The amount of time the line is pulled low determines if the bit send is a 0 or a 1.

Using a tap on pin F (PTT/Data) and E (0V) on the cable connecting the Remote Control Unit (E.C.U) to the Racal Cougar this data can be made visible on the oscilloscope.

Tap on the data cable


The following two screenshots are an example of the communication of the F line:

PWM serial data for switching a channel on the E.C.U.


Same data zoomed in (011111110)

When the line is pulled low for a short period of time a ‘0’ bit is send over the line and a longer low period send represents a ‘1’ bit. The zoomed in screenshot displays 9 bits from left to right with the following values: ’011111110’.

Timing

The duration of a bit send is about 240 µS. This makes the speed the data is send of the F line about 4.17 kbit/sec.


As mentioned above, the period of time the F line is pulled low determines if a ‘0’ or a ‘1’ bit is send. A short ‘pulse’ of about 64 µS is interpreted as a ‘0’ and a longer one of about 185 µS is a ‘1’.

'0' and '1' bit
Timing values of a '0' and '1' bit

Protocol

From what i have seen there are two types op data streams of bits, which i will call commands. There are 32 bit commands which tell the Racal to do something without additional data, i.e. changing to a different channel, and there are 52 bits commands which have the same pattern for the first 32 bits but have an additional 20 bits of data like a frequency to store for an selected channel.

32 and 52 bits command


The 32 bits of a command contain a header of 8 bits followed by an instruction of 8 bits and then repeat both bytes. So byte 1 and 3 are the same and so are byte 2 and 4.


Acknowledgement

For most of the commands send to the Racal it acknowledges the command is processed correctly by echoing the it back to the sending device after about 1000 to 1200 µS with bit 7 of the header (byte 1 and 3) set to '0'.

This can be seen in the first screenshot of this post, there seem to be two streams of data in it where the first stream is the command being send and the second stream is the command echoed back by the set.

Programming sequence

   

To program the Racal Cougar PRM 4515 the following commands are send to the set (in the order they appear below and displayed in hexadecimal notation instead of bits to save me some typing :-) ):

Init

This is a 32 bit command without any response from the set. Lacking any knowledge about this command i called it 'init'. Programming the channels will work without this command but it seems to be required to store the channel frequencies in the EEprom instead of only the processor's RAM.

Send command:  7F3B7F3B

Echoed command: None

Start


This is also a 32 bit command but with a response from the set, i assume this is a command to tell the set there is channel frequency data incoming. Programming the channels will work without this command but it seems to be required to store the channel frequencies in the EEprom instead of only the processor's RAM.

Send command:     7F8B7F8B

Echoed command: 7D8B7D8B

Channel program frequencies


For every channel to be programmed two 52 bits commands are send, one command for the receive frequency and one command for the transmit frequency. These commands can be used without the other commands described. In this case the frequencies are only stored in RAM and not in the EEprom and will be reset when powering off the set.



Send command:     7EXX7EXXXXX
Echoed command: 7CXX7CXXXXX

Details on this command are below.

Stop

The same as for the start command goes here, it is a 32 bit command with an response echoed back. I haven't figured out its use yet but this command is necessary to store the changed channels in the EEprom.

Send command:     7F037F03
Echoed command: 7D037D03

Channel frequency command

A command to set the frequency for a channel is made of 52 bits. there are 32 bits used for the header and instruction parts and 20 bits for the frequency information:

Header

The header used for the channel frequency command is 7E (HEX) , 01111110 (BIN). The header is repeated at byte 3.

Instruction 

The 8 bits used for the instruction byte contain two pieces of information: determine if the command is used to set the RX or TX frequency and the channel number the frequency is set for.

Both parts consist of 4 bits, a nibble, where the LSB (least significant or lowest value bit) is at the left side and the MSB (most significant or highest value bit) at the right. using this method (Little Endian) The decimal value 1 is represented as 1000 instead of 0001.

TX/RX

Four bits of data to set the command for receive or transmit. A value of 1 (1000 in Little Endian notation) defines the command as being for the receive frequency and a 0 (0000) defines it as being for the transmit frequency.

Channel number

The PRM 4515(L) has 10 channels available, channel 0 to 9. Four bits in Little Endian notation define the channel number the command is set for. 

Example Instruction byte for channel 7 in receive mode: 8E (HEX), 1000  1110 (BIN).

The instruction byte is repeated at byte 4.

Channel Frequency data

The last 20 bits of the Channel Frequency Command are used to define the frequency for the selected channel.

It has the following structure:

Bit 1

Parity bit for odd parity. When the number of '1' bits of the 19 following bits is even this bit will be set to 1, otherwise it will be set to 0.

Bits 2 to 4

Bits 2 to 4 store the number of times the last 0 - 87.5 Khz value of the frequency can be devided by the channel raster of 12.5 Khz. The bits are ordered in Little Endian mode which means the LSB comes first and the MSB comes last.

Example:

The last part of the frequency is 37.5 Khz. 37.5 divided by 12.5 is 3 which is is 011 binary. The value of bit 2 to 4 is 110 (Little Endian).

Bits 2 to 4 store information about the last 2 digits of the Khz part, at first i thought it was a Little Endian representation of the number of times the value could be divided by 12.5 Khz but this isn't corrent !

The correct values of bit 2 to 4 are based on the following table:


Khz.ValueHex
0 0000
12.50011
251004
37.51015
500102
62.50113
751106
87.51117

Bits 5 to 8

Bits 5 to 8 are used to store the 100 Khz digit. If the desired frequency is 70437.5 Khz then these bits will hold the binary representation of the value '4' which is 0100 in Little Endian notation.

Bits 9 to 12

Bits 9 to 5 are used to store the 1 Mhz digit. For a desired frequency at 70 Mhz these bits will hold the the binary representation of  '0' which is 0000, When the frequency would be at 71 Mhz this value would be '1' or 1000 in binary (Little Endian).

Bits 13 to 16

Bits 13 to 16 are used to store the 10 Mhz digit. For a frequency of 70 Mhz this will be 7 which is 1110 in binary (Little Endian)

Bits 17 to 20

Although not used for the PRM 4515L (Low VHF) model of the Racal Cougar these four bits are used to store the 100 Mhz digit, for example the UHF variant, the PRM 4515U where the frequencies can be programmed within the 400 Mhz range.

Example

To set channel 7 to a receive frequency of 70.425 Mhz The following 52 bit command is used:


01111110 10001110 0111111010001110 01000010 00001110 0000

 Which would be 7E8E7E8E420E0 HEX

 Whats next


I have recorded two sets of data programming the Racal Cougar using a fill gun. One with normal subtone behaviour (subtone on) and another one with the subtone turned off for every channel. I am still in the process of analyzing the commands of the subtone off fill capture and will post info about it as soon as i have done this.

The other thing is sharing a list of commands i have recorded, not only for programming the channels but also for command executed by the E.C.U. (Remote Control Unit).

In the mean time i will continue to work on the Arduino code so it is useful for anyone who needs it. 


 





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