Difference between revisions of "PTT and band switching"

Revision as of 16:22, 13 January 2018

In order to make the Portsdown transmitter integrated with transmit and receive systems, Pin 40 of the GPIO goes high when transmit is selected and the signals on pins 28 and 35 can be used for band switching.

Band outputs

These outputs are used to switch the LO filter and can be used to switch external filter and amplifiers.

Frequency Mhz	ATV Band	Pin 28	Pin 35
< 100 MHz	71 MHz	Lo	Lo
100 - 250 MHz	146 MHz	Lo	Hi
250 - 950 MHz	437 MHz	Hi	Lo
950 - 4400 MHz	23cms	Hi	Hi

Care must be taken to properly buffer these 3.3v signals from relay transients.

GPIO band decoder

As we only have 2 GPIO pins to provide 4 outputs, these outputs need to be decoded to provide an output for each band.

The Portsdown team have designed a simple board which can be either constructed on 0.1” perforated board (veroboard) or on a PCB - this can be home etched using the details below or blank PCBs are available in the BATC shop.

File:DECODE 4 Band instructions (2).pdf

File:Decode 4 Band Schematic.pdf

File:Decode 4 Band Perf Bd wiring.pdf

File:DECODE 4 Band Mirrored w SS.pdf

File:DECODE 4 Band Mirrored.pdf

File:DECODE 4 Band PCB Layout w SS.pdf

PTT steering

These band outputs can then used to "steer" the PTT line to the correct Power Amplifier using this simple relay and diode circuit - no PCB needed :-)

Tx Rx sequencer

We also recommend the use of a Tx / Rx sequencer to ensure your mast head relay has changed over and your expensive pre-amp is de-powered before you start transmitting. The sequence is as follows:

To go to Tx:

1) Turn off receiver pre-amp

2) Switch ant c/o relay to transmit

3) Apply power to PA

To go to rx

a) Turn off Tx

b) Switch ant c/o relay to receive

c) Apply power to pre-amp.

Many circuits and pre-built designs are available including this simple single chip design by Ian Waters G3KKD published in CQ-TV 125.

David M0YDH shows Ian's design in operation:- G8ADE RELAY SEQUENCER VIDEO and an updated version with extra relay for Portsdown PTT band switching.

Summary of GPIO connections

As the Portsdown project has expanded in scope, so has demand for the GPIO connections on the RPi. Here is the latest list as of January 2018:

Pin No	BCM No	Wiring Pi	RPi Name	Primary Use	Secondary Use
1	-	-	3.3v
2	-	-	5v
3	2	8	SDA 1
4	-	-	5v
5	3	9	SCL 1
6	-	-	0v
7	4	7	GPIO 7		(was DigiThin)
8	14	15	TxD		(was DigiThin)
9	-	-	0v
10	15	16	RxD		(was DigiThin)
11	17	0	GPIO 0	LCD Display	-
12	18	1	GPIO 1		(was DigiThin)
13	27	2	GPIO 2	Shutdown LED
14	-	-	0v
15	22	3	GPIO 3	Shutdown Button
16	23	4	GPIO 4	LCD Display	-
17	-	-	3.3v
18	24	5	GPIO 5	LCD Display	-
19	10	12	MOSI	LCD Display	-
20	-	-	0v
21	Example	Example	Example	LCD Display	-
22	Example	Example	Example	Example	Example
23	Example	Example	Example	Example	Example
24	Example	Example	Example	Example	Example
25	Example	Example	Example	Example	Example
26	Example	Example	Example	Example	Example
27	Example	Example	Example	Example	Example
28	Example	Example	Example	Example	Example
29	Example	Example	Example	Example	Example
30	-	-	0v
31	Example	Example	Example	Example	Example
32	Example	Example	Example	Example	Example
33	Example	Example	Example	Example	Example
34	-	-	0v
35	Example	Example	Example	Example	Example
36	Example	Example	Example	Example	Example
37	Example	Example	Example	Example	Example
38	Example	Example	Example	Example	Example
39	-	-	0v
40	21	29	GPIO 29	TX LED and PTT	-

The BCM numbers in this table correspond to the connector labelling on the Willow Electronics GPIO breakout board. Two of the connections are not broken out and need to be directly connected to the GPIO pins. More details to follow.

@@ Line 92: / Line 92: @@
 | 2 || - || - || 5v ||  ||
 |-
-| 3 || 2 || 8 || SDA1 ||  ||
+| 3 || 2 || 8 || SDA 1 ||  ||
 |-
 | 4 || - || - || 5v ||  ||
 |-
-| 5 || 3 || 9 || SCL1 ||  ||
+| 5 || 3 || 9 || SCL 1 ||  ||
 |-
 | 6 || - || - || 0v ||  ||
 |-
-| 7 || Example || Example || Example || Example || Example
+| 7 || 4 || 7 || GPIO 7 ||  || (was DigiThin)
 |-
-| 8 || Example || Example || Example || Example || Example
+| 8 || 14 || 15 || TxD ||  || (was DigiThin)
 |-
 | 9 || - || - || 0v ||  ||
 |-
-| 10 || Example || Example || Example || Example || Example
+| 10 || 15 || 16 || RxD ||  || (was DigiThin)
 |-
-| 11 || Example || Example || Example || Example || Example
+| 11 || 17 || 0 || GPIO 0 || LCD Display || -
 |-
-| 12 || Example || Example || Example || Example || Example
+| 12 || 18 || 1 || GPIO 1 ||  || (was DigiThin)
 |-
-| 13 || Example || Example || Example || Example || Example
+| 13 || 27 || 2 || GPIO 2 || Shutdown LED ||
 |-
 | 14 || - || - || 0v ||  ||
 |-
-| 15 || Example || Example || Example || Example || Example
+| 15 || 22 || 3 || GPIO 3 || Shutdown Button ||
 |-
-| 16 || Example || Example || Example || Example || Example
+| 16 || 23 || 4 || GPIO 4 || LCD Display || -
 |-
-| 17 || Example || Example || Example || Example || Example
+| 17 || - || - || 3.3v ||  ||
 |-
-| 18 || Example || Example || Example || Example || Example
+| 18 || 24 || 5 || GPIO 5 || LCD Display || -
 |-
-| 19 || Example || Example || Example || Example || Example
+| 19 || 10 || 12 || MOSI || LCD Display || -
 |-
 | 20 || - || - || 0v ||  ||
 |-
-| 21 || Example || Example || Example || Example || Example
+| 21 || Example || Example || Example || LCD Display || -
 |-
 | 22 || Example || Example || Example || Example || Example