Re: Print Servers for CNC

If one had a 3 port multiplexer as was posted, and connected 4
steppers to each, how could you switch the output to keep any 2
steppers on seperate ports syncronized ?

timeing of the switcheing is the critical problem.

if it is just a one shot switch so one could run a lathe or mill or
router, no prob. but if one is running a mill with 6 axis movement
there is little chance the switching could take place in a
transparent manner and that there was a very high degree of certanty
that there would be no lost steps.

for example, if one were threading and had spindle speed under
control and had to switch to port 2, spindle speed would be lost. It
doesn't mean that there is not a work around.


Dave



--- In CAD_CAM_EDM_DRO@yahoogroups.com, Tony Jeffree <tony@j...>
wrote:

> At 08:36 25/02/2003 -0500, Jeff Goldberg wrote:
> >I don't think speed will be an issue with a modern print server as

they are

> >generally the same throughput as a PC parallel port. The problems

may arise

> >from the ability to directly output signals on pins which are

unusual for

> >printers to use in the same way. Don't forget, they are not

directly

> >coupled to your PC, so any fooling around that the programmer does

with the

> >innards of your PC to generate or read signals may not work on a

print

> >server.
>
> I agree (see below).
>
>
> >Also, Ethernet works on a collision based retransmission basis.

On a small

> >network this probably wouldn't cause a problem, but on a network

with lots

> >of traffic flowing, your data may be slightly delayed in reaching

the print

> >server which might cause issues.
>
> You are right that the various "shared medium" variants of Ethernet

(the

> ones that used coaxial cable, or used twisted pair cables

interconnected

> via "hubs") use a collision detection/backoff/retransmission

algorithm to

> resolve contention for the medium; however, that is NOT true of the

more

> recent "full duplex" Ethernet variants (that use "switches" to

interconnect

> twisted pair segments). Switched or full-duplex Ethernet does not

employ

> collision detection/backoff/retransmission at all, and a decently

designed

> "non-blocking" switch will deliver the full available bandwidth to

all of

> its connected stations.
>
> Having said that, converting a print server into something that you

could

> use for CNC purposes sounds like hard work to me for other reasons -

mainly

> associated with the fact that they are designed on the assumption

that what

> is connected to them is a printer, which will accept data a

character's

> worth at a time, rather than the way that most parallel port-based

CNC

> controllers operate, where the controller is continuously

monitoring the

> state of individual port pins. So you would have to design whatever

you

> attached to the parallel port to perform the same handshaking

protocol that

> the printer uses to communicate with the server, and to accept data

a

> character at a time; similarly, you would have to design the

control

> software in the PC to output a stream of characters that the

controller

> could then sensibly interpret. This would potentially be further
> complicated by any spooling mechanisms that Windows inserts between

the

> virtual parallel port on the PC and the print server, which could

insert

> delays that could be far more significant than any that a shared

medium

> Ethernet LAN might impose.
>
> Also, communicating data in the other direction (for example, limit

switch

> contact closures) would be tricky.
>
> Regards,
> Tony
>
>
> >I'd tend to suggest sticking to popular name brands because driver
> >availability might be an issue if you change operating systems in

the future

> >(unless the price difference is too great, and you get involved up

front

> >with the assumption that it may end up as a paperweight sometime

in the

> >future)
> >Theoretically, they should work, from a practical standpoint,

you'd have to

> >try it.
> >
> >Jeff
> >
> >
> >-----Original Message-----
> >From: ccq@x... [mailto:ccq@x...]
> >Sent: Tuesday, February 25, 2003 7:28 AM
> >To: 2ccq@x...
> >Subject: [CAD_CAM_EDM_DRO] Print Servers for CNC
> >
> >
> >The following URLs have a print server on them; That's a 10/100

ethernet

> >port feeding three parallel ports. Would these be usable for CNC

work?

> >Would the software be able to access them properly? I would think

they'd be

> >speedy enough for smaller, slower machines, but.....
> >
> > http://www.pcbay.net/gen3parpor10.html
> > http://www.amastore.com/gen3porparpr.html