[LinuxPPS] Trying to get parallel port client working on 2.6.26

[Hal V. Engel]

On Friday 19 September 2008 10:54:17 am Paul Simons wrote:
> I have an old TrueTime 4<mumble>-DC that uses a GPS to GOES converter.
> I've been a member of pool.ntp.org for some time now, and I'd like to get
> more accurate and get PPS running.  This clock puts out a TTL PPS.  First
> question:
>
> I notice that people seem to convert from TTL to RS232 and then use the
> serial port.  Why is that?

My OnCore UT+ has a 5V TTL PPS signal and I just run it directly to the DCD 
pin of my serial port.  It works fine and I supect that this is how most users 
with a TTL PPS signal handle this athough some may run it through an inverter.  
I don't think most people use a TTL to RS232 converter for the PPS signal.  

>
> I am using the parallel port (pin 10 - ACK, pin 24 - ACK ground) and just
> going straight TTL.  Everything seems to register fine:
>
> dmesg | grep -i pps
> LinuxPPS API ver. 1 registered
> PPS line discipline registered
> new PPS source parport0 at ID 0
> parport_pc 00:08: PPS source #0 "/dev/lp0" added
> dmesg | grep -i parport
> parport_pc 00:08: reported by Plug and Play ACPI
> parport0: PC-style at 0x378 (0x778), irq 7, dma 3
> [PCSPP,TRISTATE,COMPAT,ECP,DMA]
> lp0: using parport0 (interrupt-driven).
> new PPS source parport0 at ID 0
> parport_pc 00:08: PPS source #0 "/dev/lp0" added
>
> However, no joy:
>
> ./ppstest /dev/pps0
> trying PPS source "/dev/pps0"
> found PPS source "/dev/pps0"
> ok, found 1 source(s), now start fetching data...
> time_pps_fetch() error -1 (Connection timed out)
> time_pps_fetch() error -1 (Connection timed out)
> time_pps_fetch() error -1 (Connection timed out)
> ^C
>
> I am using a Debian kernel:
>
> uname -a
> Linux mercury 2.6.26.simonet #1 SMP Sat Sep 13 11:10:50 PDT 2008 i686
> GNU/Linux
>
> and I am using the ntp-pps-2.6.26.diff patch.
>
> I'd like to see this working as a first step.  Is that reasonable?  

This should be OK.

> Should
> I start sprinkling printks to see where I'm going wrong?  I hooked up a
> digital multimeter and it twitches;

This is what I get with a nultimeter with the PPS signal.

> should I get some sort of LED?  

I run the TTL PPS signal through two inverter stages and a resister to an LED 
to isolate the LED from the PPS circuit.  I found this reassuring when I first 
hooked things up since I can see when the PPS signal is active.  But if your 
multimeter twitches you should be OK.   

> Should
> I convert to RS232 signaling?

No you shouldn't need to.  Almost all serial ports now days will function just 
fine when the DCD pin is pulled high at least if the TTL voltage is 5 volts (I 
don't know if this is the case with a 3 volt TTL but it might work as well).

Since I use a serial port I don't know how much different things are for a 
paraport.  Did you run ppsldisc /dev/lp0 during the time when you were running 
ppstest?   If not IRQs from the PPS device will be masked.  Also only one 
application at a time can be using the PPS signal so make sure you don't have 
the PPS reclock driver running at the same time as ppstest.

>
> Okay, I'll stop now.