[time-nuts] SRS PRS10 repair

Magnus Danielson magnus at rubidium.dyndns.org
Tue Aug 25 17:59:22 EDT 2015

Hang on a minute, polarity does not switch all of a sudden.

However, a short or a glitch could cause the signal to be garbled such 
that we incorrectly interpret it as inverted. It can also be the result 
of the signal 0 to 5V being triggered on the 0V line on the wrong transient.

It might be that you have a perfectly good signal, but your RS232 can't 
interpret it correctly.

So, check the signal, if it has nice digital shape, check if it is only 
a matter of RS232 level error. You might need to convert it using a 
level shifter. The RS232 trick being used may not be tolerated by all 
RS232 inputs.


On 08/25/2015 07:35 PM, Brian M wrote:
> The earlier suggestion of a missing inverter seems to be the right thing to
> chase this evening. I was able to add an inverter and decode the first few
> characters on a scope. I get the expected DC1-CR-P-R-S sequence.
> Thanks for the input on this. I'll reply back after I've had more time to
> hack at this.
> - Brian
> On Tuesday, August 25, 2015, Brian Inglis <Brian.Inglis at systematicsw.ab.ca>
> wrote:
>> Hi,
>> You have too many 1s in your startup string compared to the expected
>> "PRS_10\r".
>> If the MCU clock is not 10Mhz then the integrated UART rates will be off,
>> which should produce framing errors, but do UARTs still detect and systems
>> report these nowadays, or just pass along garbled data?
>> Otherwise, garbled data is most often a result of inadequate pin contact,
>> if the connectors are not seated properly, or the pins or sockets are loose
>> in their shells.
>> Age and rough treatment can have that effect.
>> "Internal hardware jumpers allow these pins to be configured as analog
>> outputs
>> to monitor the lamp intensity and varactor voltage for complete
>> compatibility
>> with the FRS."
>> Have you checked the jumpers in the manual Configuration Notes:
>> "Pin 4: TXD/PHOTO The default configuration uses this pin as an output for
>> RS-232 data.
>> Many system parameters (including the lamp intensity) may be monitored via
>> the RS-232
>> interface. The function of this pin may be changed to an analog monitor
>> for the lamp
>> intensity by removing one resistor (R347) and installing a 10 kΩ resistor
>> for another (R348)
>> on the microcontroller PCB."
>> On 2015-08-24 22:40, Brian M wrote:
>>> I tried through the weekend, double and triple checking wiring and setup.
>>> I've tried the following methods of getting serial comms working:
>>> PRS10 -> Arduino Uno (with processor bypassed) -> USB Host
>>> PRS10 -> Level Shifter -> BBB UART
>>> PRS10 -> MAX232 -> USB Serial adapter
>>> Shortly after power is applied to the PRS10, I do get a string of
>>> characters. Believe it should be the model information. Instead I get:
>>> wy+VPgy
>>> I guess the good news is that this output appears consistent with each
>>> power cycle of the device. And I'm getting the same results through all
>>> the
>>> hookup methods I've tried.
>>> My minicom settings are for software flow control at 9600 8N1 - from what
>>> the manual states, this should be the right settings. I've tried screen as
>>> well - and get the same text. I went crazy trying several other rates and
>>> setting combinations. No luck.
>>> Maybe I've missed something obvious.
>>> I agree that getting comms going to the MCU are going to be an important
>>> step. How do people address this type of problem? Scope the serial and try
>>> to decode by hand? The 10Mhz to the MCU looks OK on a scope. Are there
>>> further steps people try after that? If nothing else I think there's some
>>> interesting stuff to learn here. I also wouldn't mind tearing out the
>>> electronics, determining if the lamp is good, and attempt to build from
>>> there. I don't know the datecode for the unit, the PCB is marked with a
>>> datecode suggesting 2003? I don't have the full case. I'm trying to assess
>>> what are reasonable next steps. How do I determine if the MCU is healthy?
>>> If the MCU is fried, how do I determine if I just need to squeeze a new
>>> MCU
>>> board in there?
>>> Thanks! I appreciate the input so far!
>>> - Brian
>>> PS - after looking again at the signal on the scope, it does seem like it
>>> is 9600 baud. ~100µS per bit. The data out on the MCU itself looks like
>>> what I saw on the main connector.
>>> On Sat, Aug 22, 2015 at 2:04 PM Mike Cook <michael.cook at sfr.fr> wrote:
>>>> Le 22 août 2015 à 03:40, Bob Camp <kb8tq at n1k.org> a écrit :
>>>>> Hi
>>>>> On any microprocessor based gizmo, getting the micro running (again) is
>>>>> generally priority number one. It sets everything up and gives you the
>>>> diagnostic
>>>>> info you need to go further. Garbled serial is better than none at all.
>>>> It suggests
>>>>> something short of a total MCU death spiral …
>>>>> Bob
>>>>> On Aug 21, 2015, at 7:26 PM, Brian M <brayniac at gmail.com> wrote:
>>>>>> Dear list -
>>>>>> I have come into possession of a for parts prs 10. I'd like to try to
>>>>>> repair this device. What I've noticed so far. Serial is garbled. (Even
>>>>> at
>>>>> varying baud rates).
>>>>    You don’t say how you are connecting to the Rb. The manual states:
>>>> "RS-232 data is sent to the host on pin 4, received from the host on pin
>>>> 7. The baud rate is
>>>> fixed at 9600 baud, 8 bits, no parity, with 1 start and 1 stop bit. No
>>>> DTR
>>>> or CTS controls are
>>>> used; rather, the XON/XOFF protocol has been implemented. The transmit
>>>> drive level is 0
>>>> and 5 V, not the +/-12 V normally associated with RS-232. These levels
>>>> are
>>>> compatible with
>>>> most RS-232 line receivers, but does not require their use (a TTL
>>>> inverter
>>>> may be used
>>>> instead), hence simplifies the interface when used inside an instrument
>>>> at
>>>> the sacrifice of
>>>> degraded noise immunity over long lines."
>>>> So make sure that you adhere to that.
>>>> Lamp isn't lit.
>>>> What’s the date code. Early versions may be reaching EOL, though 20yrs id
>>>> quoted.
>>>> Doesn't look great. I'd like to know
>>>>>> if anybody else has wandered down this path. What are common failure
>>>>> modes?
>>>>> Anything match up with what I describe? Voltages to check would be
>>>>> helpful.
>>>>> The 10MHz out looked okay on a scope. Haven't gone further yet. I
>>>>> suspect
>>>>> the crystal is fine.
>>>>>> Thanks in advance. Happy hacking!
>>>>>> - Brian
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