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From: ar12@prism.gatech.EDU (REGISTER,ANDREW H)
Newsgroups: comp.periphs.printers
Subject: Re: How fast is a parallel port?
Summary: 8255 PPI
Keywords: Centronics, Drivers, Hardware, Dataproducts,
Message-ID: <22865@hydra.gatech.EDU>
Date: 26 Feb 91 18:38:14 GMT
References: <1991Feb25.220636.23155@cbnewsh.att.com> <14740@sunquest.UUCP>
Organization: Georgia Institute of Technology
Lines: 34

In article <14740@sunquest.UUCP>, spg@alpha.sunquest.com (Steve Gibbons) writes:
> In article <1991Feb25.220636.23155@cbnewsh.att.com>, wcs@cbnewsh.att.com (Bill Stewart 908-949-0705 erebus.att.com!wcs) writes...
> #I'm trying to find out how to connect big laser printers (50-100ppm)
> #to normal computers, in particular to 386/486 boxes running UNIX.
> #For a variety of ugly reasons, I don't want to use Ethernet -
> #I'd really like to use a parallel port.  But nobody I've talked to
> #knows how fast a parallel port can go on a typical machine, and
> #nobody's tried it at speeds over about 10ppm.
> 
> #50-100 ppm is 1-2 pages/second, or typically 40-80 kilobits/second;
>

In the old days most of the parallel ports were made from an Intel 8255 programmable peripheral interface.  Now a`days, many are integrated in the vlsi chip sets.  Anyway I pulled the data sheet and if I am decifering the timing diagram correctly for mode 1 strobed output I get somewhere in the neighborhood of 1000 to 1500 nS per write cycle.

This is from the 80x86 write going active to the end of data going out of the i8255 and onto the peripheral.  Of course this assumes that the peripheral acks immediately.  This corresponds to a *very* large data rate.

Given this, the port is most likely software limited.  If you want to figure out the max rate then you would have to figure out the most efficient assembly code and count cycles.  Something like

beginloop:
mov ax,[dx]
mov "port address",ax
inc dx
jmp beginloop

please forgive my coding, it has been a long time since I had to resort to assembly.  And I don't remember if you have to poll the ack line.  I think that you do.

Toodles
Andy

-- 
Andy Register  Internet: ar12@prism.gatech.edu   Bitnet: aregiste@gtri01.bitnet

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