In a message dated 05/01/2008, ian@........... writes:

Hi,
actually, I don't think I'm near any limits.  The A/D I use  can handle 20K 
samples/sec. 
Hi Ian,
 
    What ADC are you using? 20 K SPS is 50 micro sec /  sample. My ADC takes 
20 muS/S.
    How does it's accuracy depend on it's sample  rate?
    Does it have an onboard processor chip to take and  average multiple 
samples?
    What data rate does the ADC board to computer link  support?

I only  have 2 instruments (though I sample each on 3 A/D channels to get the 
 resolution up to 22 bits), so that's only 6 x 50, or 300 samples/sec.   
You need to average four samples to get 1  additional bit of accuracy, 16 
samples to get two bits extra, etc.

So I  could connect up all of the A/D's 16 input channels and still not 
stress  it.
    This is likely to be ~40 bytes total with the  overheads at 50 SPS, say 
16 K bits/ sec. If you are using a 24 bit ADC, it is  likely double this. Then 
it largely depends on whether you are sending  datapackets, or individual 
bytes. 
    If you are sending asynchronous bytes, you have to  wait for the signal 
to be transmitted, the receiving server to respond and the  ACK to be received. 
Transmission delays can be significant. 
    I am 12 km from the phone terminal, so the delay  would be well over 80 
micro sec per byte. Coupled to a a 56 K modem, I certainly  could not transmit 
this much data.

The data  server does burn up 80% of the PC's CPU but it's only an old 800 
MHz machine  and wouldn't cost much to replace with one twice the speed.  Data 
across  the network connection is only 32 characters x 50 or 1600 bytes/sec.   
Less than a 500th of the 100 Mb/s network bandwidth 
(being  generous).


So, do you know what minimum speed you  can actually get for asynchronous 
transmissions? The broadband data rates  quoted by the service providers are 
usually maximums in the best possible  conditions, not the average and certinly 
not guaranteed. They may not allow  for transmission delays. Reality may be only 
a small fraction of the specs  advertised! A recent BB survey in the UK 
suggested a far lower preformance,  nearer to that of the 56 K modems. 
 
    Regards,
 
    Chris Chapman



   





In a message dated 05/01/2008, ian@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>Hi,
actually, I don't think I'm near any limits.  The A/D=
 I use=20
  can handle 20K samples/sec. 
Hi Ian,
 
    What ADC are you using? 20 K SPS is 50 micro se=
c /=20
sample. My ADC takes 20 muS/S.
    How does it's accuracy depend on it's sample=20
rate?
    Does it have an onboard processor chip to take=20=
and=20
average multiple samples?
    What data rate does the ADC board to computer l=
ink=20
support?
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I only=20
  have 2 instruments (though I sample each on 3 A/D channels to get the=20
  resolution up to 22 bits), so that's only 6 x 50, or 300 samples/sec. =
;=20
  
    You need to average four samples to get 1=20
additional bit of accuracy, 16 samples to get two bits extra, etc.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>So I=20
  could connect up all of the A/D's 16 input channels and still not stress=20
  it.
    This is likely to be ~40 bytes total with the=20
overheads at 50 SPS, say 16 K bits/ sec. If you are using a 24 bit ADC, it i=
s=20
likely double this. Then it largely depends on whether you are sending=20
datapackets, or individual bytes. 
    If you are sending asynchronous bytes, you have=
 to=20
wait for the signal to be transmitted, the receiving server to respond and t=
he=20
ACK to be received. Transmission delays can be significant. 
    I am 12 km from the phone terminal, so the dela=
y=20
would be well over 80 micro sec per byte. Coupled to a a 56 K modem, I certa=
inly=20
could not transmit this much data.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>The data=20
  server does burn up 80% of the PC's CPU but it's only an old 800 MHz machi=
ne=20
  and wouldn't cost much to replace with one twice the speed.  Data acr=
oss=20
  the network connection is only 32 characters x 50 or 1600 bytes/sec. =
=20
  Less than a 500th of the 100 Mb/s network bandwidth 
(being=20
  generous).


    So, do you know what minimum speed you=20
can actually get for asynchronous transmissions? The broadband data rat=
es=20
quoted by the service providers are usually maximums in the best possible=20
conditions, not the average and certinly not guaranteed. They may not a=
llow=20
for transmission delays. Reality may be only a small fraction of the specs=20
advertised! A recent BB survey in the UK suggested a far lower preformance,=20
nearer to that of the 56 K modems. 
 
    Regards,
 
    Chris Chapman