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Futaba R127DP Rx: PCM1024 *AND* PCM512?

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Old 02-17-2007, 11:50 AM
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Little-Acorn
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Default Futaba R127DP Rx: PCM1024 *AND* PCM512?

I have two older Futaba PCM transmitters, and two old Futaba R127DP receivers. One Tx is a PCM1024 type, the other is a PCM512 type. (PCM512 was an early version of PCM, that was phased out when PCM1024 became the standard that mosts PCM systems use today. PCM512 and PCM1024 are not compatible with each other, and only a few PCM512 systems were made).

I have seen Futaba literature that says the R127DP receiver is the only one that will work with both 512 and 1024. (There is an R128DP receiver that is PCM512 only, I don't have one). Most newer Futaba PCM receivers are PCM1024 only.

Both my R127DP receivers work correctly with my PCM1024 transmitter. They power up, move their servos as expected, etc. Neither works with the PCM512 transmitter, the servos stay dead. (The two transmitters are on Ch52 and Ch56, I swap crystals in the receivers appropriately). Maybe something is wrong with the PCM512 transmitter, I suppose. But it gets a good power indication on its meter, the lights flash just the way the Tx manual says they should when I power up, and I put an oscilloscope on the antenna and found a 72.830 MHz wave (correct for Ch52), amplitude around 20V p-p, which sounds pretty hefty to me. I'd like to think the 512 Tx is working correctly.

My question is, Does the R127DP receiver really decode both PCM versions? Can it switch automatically from one to the other, depending on what it hears? Or do you have to open the receiver case and flip a switch or change a jumper somewhere? Or does Futaba literature mean that SOME R127DPs were made for PCM512, while other R127DPs were made for PCM1024, and you still can't use one for the other?

Any advice gladly accepted!
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Old 02-17-2007, 03:42 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

acorn,
I have two cirrus CR227A PCM receivers made by Futaba and I beleive that they are rebadged R127DP units. They have the same case etc. I opened one up to look at the decoder and it is the same decoder chip used in the R129DP. They both work great with PCM1024. That is all that I am interested in anyway. I don't see how these decoder chips could automaticly switch between 512 and 1024. The R129DP can't. They probably made two version, just like the R129DP. Mine have the red dot in the crystal well denoting the new decoder.

Dave
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Old 02-17-2007, 08:37 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Since you have an oscilloscope there, have you tried pulling out the transmitter module, and checking the pins to see if you have the PCM signal on one of them?
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Old 02-17-2007, 11:59 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Look at the last letter in the Rx designator....127DF. The "F" signifies FM.

(conversely, a 129DP, 138DP, 149DP, etc all denote PCM receivers)

The Futaba 127DF is an FM (PPM) receiver. It doesn't decode any PCM signal. 512, 1024, 2048 or any other.

By selecting the correct mode of modulation at the TX (PPM), your PCM TX will then work fine with the FM Rx, but it will NOT function as a PCM system one iota if the 127DF receiver is involved.





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Old 02-18-2007, 01:39 AM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Futaba did make an R127DP that was discontinued around 1994 - it was suppose to be compatible with both 512 and 1024 PCM according to the Futaba website (you have to search around a little to find the listing where it says that it is switchable between 512 and 1024, but it is there).

If you look in the list of discontinued receivers, it is listed as narrow band 512 PCM.
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Old 02-19-2007, 12:38 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

if the 127DF receiver is involved.
It isn't. In my original post I said R127DP, six times. Not sure where the talk about the R127DF is coming from.

I've heard from some of the Futaba factory personnel, that the R127DP will switch between PCM512 and PCM1024 automatically, the operator doesn't have to flip any switches, change any jumpers, or anything else, in the receiver or transmitter.

The fact that both my R127DPs work perfectly with my PCM1024 transmitter, but not at all with the PCM512 transmitter, indicates that something is wrong with the PCM512 transmitter.
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Old 02-19-2007, 08:06 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

ORIGINAL: jdb000001
Since you have an oscilloscope there, have you tried pulling out the transmitter module, and checking the pins to see if you have the PCM signal on one of them?
Excellent idea! I swiped\\\\\\borrowed a scope from work and did a little probing today. I used it on one of the wires going to the RF module in the T8SSA-P (the orange wire, if that tells anybody anything, it has no other label). I found something very PCM-looking on it. But I see a few oddities, at least they look that way to me. This is only the second time I've ever scoped an RC transmitter, the first being a 3-channel Ace digital Tx about 25 years ago.

I do troubleshooting of digital circuits (computers running at multi-MHz speeds) for a living, so I'm familiar with many of the factors involved. But without a schematic diagram for this circuit, I'm a little nervous to do anything aggressive here.




The first shot shows the pulse train. It's taken with a sweep speed of 2.5 ms/division. I triggered on a low lasting 3 ms or more. This produced a very repeatable pattern, looks like the 3ms low is the sync pulse.

First thing that struck me, is that the pulses aren't square. Most digital pulses I've seen, are square, especially at millisecond speeds like this. Should these be square? If not, it's a strange way to run a railroad.

The second scope shot shows the same pulse train. I reset the sweep speed to 0.5 ms/division, and captured the pulse train again. When the signal goes low, it does so very quickly, as I would expect. But when it goes high, it does so in a very leisurely fashion, and seems to pause at about 2.2V. Then it eventually gets up to 5V.

If this were a normal digital circuit like those I'm used to troubleshooting, my first impulse would be to add a 1K ohm resistor between this wire and +5V, to try to get the pulse to rise faster and become more or less square. Anybody know a good reason why I shouldn't do this? WIll I get smoke? I don't have a schematic for this circuit, so it's hard to tell.

Most digital circuits use an active transistor to pull the signal to ground, resulting in the kind of very fast fall you see here. When it's time for the signal to go high, some circuits use a resistor attached to the higher voltage (+5V) to pull it back up, once the transistor releases its pull-down influence. Other circuits use a second transistor to pull the voltage high. Looks to me like this circuit isn't doing much of either.

It's especially strange how the voltage starts to rise from 0V, and then seems to pause around 2.2 or 2.3 volts, before rising the rest of the way. Could it be that there's supposed to be a transistor pulling the voltage up, but the transistor is sick? Or maybe the transistor that did the pulling down, is reluctant to stop pulling when it's told to?

Anybody familiar with the T8SSA-P transmitter? Or Futaba digital Tx's in general? Do these scope shots tell you anything useful?
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Old 02-20-2007, 02:36 AM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Just looked at the signal on the module pin of an 8UAP - seems to be a very low level signal of about 50mV with no module installed, but fairly square-wave looking. The signal out the trainer jack is about 4.5V and nice and squared off. Didn't try it with the module installed because I didn't want to take the back off the radio.
Take a look at the trainer jack and see if you have a good signal out of that - that would at least let you know if the signal is getting that far. Could be something as simple as a bad trainer switch not letting the signal get thru.

I've never had the opportunity to play with an 8SSAP - its a rather rare and quite valuable little radio.

You are using the proper module - an FP-TF-FM - aren't you?

Don't know where you would find any schematics for the radio - Futaba and most of the other manufacturers are very hesitant to let anyone get a hold of those.
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Old 02-20-2007, 11:42 AM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Hmmmm, jdb, the trainer switch is, in fact, bent. It's the closest anything on this transmitter has come, to being damaged. I was wondering if that had any effect. But I had assumed it would be an all-or-nothing thing. And since I was getting a signal, I thought the switch must be OK. But you have made me think.

I might wire around that trainer switch and see if it changes anything.

Excellent tip!

If it changes nothing, I might try the resistor-pullup trick. Might start with a 10K to +5V just in case, considering how slow the speeds are relative to what I'm used to. And will probably mount it on the encoder board rather than the plugin to the RF module, closer to the source of the signal.

BTW, the ground for the RF module seems to float free, away from the chassis ground (if any). Is this normal?

The RF module is the one that came with the transmitter, don't know what number. Yet. I'll check if I have time.
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Old 02-20-2007, 12:40 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Well, I took a quick look at my parts radio, and the signal line to the RF module appears to be driven thru a transistor, so is probably open collector - that would explain my low single level with no module installed.

The ground for the RF module on the 8U is the (-) battery ground plane - all the lines to the module plug, and the trainer jack for that matter, go through inductors to keep the RF out of the lines. The case on my radio is mostly plastic, so no grounding there.

Does the 127DP have a connection for the DSC cable? That would at least let you know if it would accept the 512 PCM coding.
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Old 02-20-2007, 12:47 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Hmmm, another excellent question about the DSC cable, and I don't know the answer. That one goes on the to-do stack right after the trainer switch, the resistor, and thirty honeydo's.

From what I recall seeing on the R127DP, it has eight slots for servo-type plugs. One is labeled B (for battery I guess), the others numbered 1 thru 7. Then there is the slot for the crystal, and that's all I recall seeing. But I'll look again, now that I know what to look for.
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Old 02-22-2007, 03:29 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Well, I just talked to the fellow at RadioSouth, and he said a few interesting things:

1.) The transmitter takes 8 nicad cells, not nine. Mine came with nine, all very old and leaky. I'd never seen a nine-cell Tx pack, and checked carefully to see if one of them was a dummy. But they all looked identical, so I replaced them with nine Sanyo 700mAh cells. Uh-oh. Recipe for toast, well done?

2.) A Futaba R127DP receiver will not, repeat not work with the PCM512 transmitter such as this T8SSA-P. I know, Futaba's website says it works with both PCM512 and PCM1024 transmitters. And at least one radio expert on this forum has said the R127DP will work with the PCM512 Futaba transmitter. This fellow at RadioSouth says flatly they are wrong. The ONLY receivers the T8SSA-P will work with, are the narrowband R128DP, and the older wideband R118GP. He insisted on this, and sounded very sure of his facts. Hmmm. All I can tell you is, my two R127DP receivers, worked fine with another PCM1024 transmitter I have, and have never worked with the T8SSA-P. So the weight of evidence, what there is of it, seems to be on this fellow's side.

He also said that when you replace a battery in this transmitter, you must reset the voltage regulator by turning the Tx on, then off, then on again. If you don't, you'll fry something. I didn't. Damn....

This story is a long way from over.
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Old 02-23-2007, 10:12 AM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Well, did you ever look to see if you were getting a good PCM signal out of the trainer jack or DSC jack?

Never heard of a voltage regulator that had to be reset, but guess there's always a first time.

Just out of curiosity, what model is the PCM1024 transmitter you are using to test the 127DP's?

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Old 03-19-2007, 01:40 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

Well, hush my mouth. I finally got a Futaba R128DP receiver - one of the few designed for PCM512 transmitters only. Plugged in the servos, battery, and crystal, and IT WORKED with the transmitter!

I was VERY pleased... and surprised! Finally I have a working single-stick 8-channel system!

Now, what plane should I put it in......... :^)
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Old 05-06-2007, 10:08 AM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

I have been told that the Futaba R127DP receiver will work with my FP-T6NPK Transmitter. I was wondering if you still had yours, and if you would be interested in selling it. Thanks.
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Old 06-04-2007, 05:49 PM
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Default RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?

I found this chart on the back of a flying models magazine dated 1989. The wave forms advertised might be of interest.
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