RA1792 uController interface - UK Vintage Radio Repair and Restoration Discussion Forum

walter pe1ful · 30th Mar 2019, 11:11 pm

I obtained my Racal RA1792 earlier last year that had uController problems. At that time one was able to enter frequencies that were actually displayed but not tuned.

During trouble shooting the uController stopped running at all. Perhaps the leakage of the backup battery finished its job. So in order to speed up investigations I built a Raspberry Pi drop in replacement for the uC board. A couple of 3V3 to 5V level converters on a Vero board with connectors to front panel display board and the Pi.

With help from the RA1792 maintenance manual and the Service manual I am able to read out the keyboard matrix and the rotary encoder. However writing the LCD display still isn't working. Digits appear but more or less randomly. Controlling the synthesizers and other circuits still to be explored.

So my question is if anyone has done something similar before or knows a source for more detailed information about timing of the bus signals than the textual description from the mentioned manuals? Especially the write strobe versus data / control lines is unclear.

73 Walter

frsimen · 2nd Apr 2019, 10:00 am

Hello Walter,

Welcome to the forum. I have developed a replacement CPU board for my Racal 1792 based on an ARM Cortex M0 development board. I encountered the same problem as you with the random numbers/symbols appearing on the display.

The original CPU in the Racal is relatively slow and works well with the CMOS devices on the front panel board. The ARM processor I am using is a lot faster and the code I wrote needed some time delay loops to be added whenever a write or a read was made to/from the Racal. In my case, the delays used were of the order of 10µs and that completely overcame the random characters problem.

Your Raspberry Pi is faster still but the cure will be the same.

Once the delays are in place, the sequence described in the manual will work well. So long as the delays are sufficiently long, the timings of the data, addressing and write strobe are not critical.

Be aware that there are many different versions of the synthesiser, IF amplifier/AF amplifier and front panel boards. I used the same method as you, working from the service manual. I couldn’t get the AGC switching to work, as the address and bit allocations used on my board were different to the one in the service manual I was using.

The service manual for the RA6790 is also very useful. It gives a better description of the operation of some of the circuitry, which is similar to, but not identical to, that of the RA1792.

Paula

walter pe1ful · 2nd Apr 2019, 11:12 am

Hi Paula,

Very interesting to hear you've already performed a similar approach. I had to choose between MSP430, Arduino and Raspberry PI and having Linux inside sounded challenging. I hope to repair the original processor board, but in the mean time wanted to verify the other parts of the receiver.

My first attempts were in Python, next step in plain C, with similar results. So I wonder if speed is the issue, but I will try. Another point of worry is the level converters I have used. When I studied the circuits in more detail I found that the IOD0-7 data lines use a 10k pullup and the IOC0-7 10k pull down resistors. Perhaps the TXB108 I have chosen prefer weaker pullups / pulldowns. It seems the TXS108 is more suitable. I need to study the signals in more detail on the TTL side.

Perhaps you can tell me how you have taken care of the level conversion?

Kind regards,
Walter

frsimen · 2nd Apr 2019, 2:04 pm

Hello Walter,

The ARM microcontroller I used had 5v tolerant I/O pins, so all I needed was a pair of LS TTL bus drivers and a hex inverter.

I used C for my efforts, which has given satisfactory results.

Paula

walter pe1ful · 2nd Apr 2019, 8:43 pm

Hi Paula,

It seems you're quite right about the timing. Enlarging the write strobe to a 100ms gives more correct digits on the display. Still many errors though.

I had read some annotation in the schematic at the strobe line that said 500ns for reference. But a much longer strobe seems more reliable.

Next thing to do is observing the level converter outputs on the oscilloscope in detail. But that'll have to wait for the weekend.

Many thanks for your help,
Regards,
Walter

30th Mar 2019, 11:11 pm	#1
walter pe1ful Diode Join Date: Mar 2019 Location: Druten, Netherlands. Posts: 3	RA1792 uController interface I obtained my Racal RA1792 earlier last year that had uController problems. At that time one was able to enter frequencies that were actually displayed but not tuned. During trouble shooting the uController stopped running at all. Perhaps the leakage of the backup battery finished its job. So in order to speed up investigations I built a Raspberry Pi drop in replacement for the uC board. A couple of 3V3 to 5V level converters on a Vero board with connectors to front panel display board and the Pi. With help from the RA1792 maintenance manual and the Service manual I am able to read out the keyboard matrix and the rotary encoder. However writing the LCD display still isn't working. Digits appear but more or less randomly. Controlling the synthesizers and other circuits still to be explored. So my question is if anyone has done something similar before or knows a source for more detailed information about timing of the bus signals than the textual description from the mentioned manuals? Especially the write strobe versus data / control lines is unclear. 73 Walter

2nd Apr 2019, 10:00 am	#2
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: RA1792 uController interface Hello Walter, Welcome to the forum. I have developed a replacement CPU board for my Racal 1792 based on an ARM Cortex M0 development board. I encountered the same problem as you with the random numbers/symbols appearing on the display. The original CPU in the Racal is relatively slow and works well with the CMOS devices on the front panel board. The ARM processor I am using is a lot faster and the code I wrote needed some time delay loops to be added whenever a write or a read was made to/from the Racal. In my case, the delays used were of the order of 10µs and that completely overcame the random characters problem. Your Raspberry Pi is faster still but the cure will be the same. Once the delays are in place, the sequence described in the manual will work well. So long as the delays are sufficiently long, the timings of the data, addressing and write strobe are not critical. Be aware that there are many different versions of the synthesiser, IF amplifier/AF amplifier and front panel boards. I used the same method as you, working from the service manual. I couldn’t get the AGC switching to work, as the address and bit allocations used on my board were different to the one in the service manual I was using. The service manual for the RA6790 is also very useful. It gives a better description of the operation of some of the circuitry, which is similar to, but not identical to, that of the RA1792. Paula

2nd Apr 2019, 11:12 am	#3
walter pe1ful Diode Join Date: Mar 2019 Location: Druten, Netherlands. Posts: 3	Re: RA1792 uController interface Hi Paula, Very interesting to hear you've already performed a similar approach. I had to choose between MSP430, Arduino and Raspberry PI and having Linux inside sounded challenging. I hope to repair the original processor board, but in the mean time wanted to verify the other parts of the receiver. My first attempts were in Python, next step in plain C, with similar results. So I wonder if speed is the issue, but I will try. Another point of worry is the level converters I have used. When I studied the circuits in more detail I found that the IOD0-7 data lines use a 10k pullup and the IOC0-7 10k pull down resistors. Perhaps the TXB108 I have chosen prefer weaker pullups / pulldowns. It seems the TXS108 is more suitable. I need to study the signals in more detail on the TTL side. Perhaps you can tell me how you have taken care of the level conversion? Kind regards, Walter

2nd Apr 2019, 2:04 pm	#4
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: RA1792 uController interface Hello Walter, The ARM microcontroller I used had 5v tolerant I/O pins, so all I needed was a pair of LS TTL bus drivers and a hex inverter. I used C for my efforts, which has given satisfactory results. Paula

2nd Apr 2019, 8:43 pm	#5
walter pe1ful Diode Join Date: Mar 2019 Location: Druten, Netherlands. Posts: 3	Re: RA1792 uController interface Hi Paula, It seems you're quite right about the timing. Enlarging the write strobe to a 100ms gives more correct digits on the display. Still many errors though. I had read some annotation in the schematic at the strobe line that said 500ns for reference. But a much longer strobe seems more reliable. Next thing to do is observing the level converter outputs on the oscilloscope in detail. But that'll have to wait for the weekend. Many thanks for your help, Regards, Walter