Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!sdd.hp.com!hp-pcd!hpfcso!rjn
From: rjn@hpfcso.HP.COM (Bob Niland)
Newsgroups: comp.sys.hp
Subject: Re: LaserJet III problems with XON/XOFF limitations
Message-ID: <7370245@hpfcso.HP.COM>
Date: 2 Nov 90 01:02:09 GMT
References: <917@usage.csd.unsw.oz.au>
Organization: Hewlett-Packard/FSY Ft.Collins,CO,USA
Lines: 53

re: > The problem is that when the LaserJet's input buffer becomes full
    > (which appears to be after 60000 bytes) it sends a control-S to the
    > DECstation, but the DECstation can transmit up to another 40 or so
    > characters before stopping.  Unfortunately, the LaserJet only appears
    > to be able to buffer another 30 characters before it gives an IO
    > CONFIG ERR.

If the DEC is indeed sending 40 chars, I can believe this is happening.

> My local HP response centre have not been particularly helpful and
> assure me that the problem is with the DECstation.

That would be my conclusion as well.  There is no formal spec for Xon/Xoff
handshaking.  After HP got its fingers burned on the 2225D serial ThinkJet,
which has only an 8 byte FIFO, we reportedly determined that industry
practice is 32-byte Xoff hysteresis.  Either end may send up to 32 more
bytes after receipt of an Xoff.  I would expect the LaserJet to have a
32-byte FIFO, but probably not more.

I'm really surprised that DEC ships 40 bytes.  For all practical purposes
DEC invented Xon/Xoff, so they should know better.  Some HP workstations
allow you to configure the FIFO on each serial port.  Does DEC have some
similar capability in the device file minor number, an ioctl call or as an
'stty' option?

> However, I have used the DECstation with Apple LaserWriters and NEC LC890s
> without difficulty.  Perhaps HP's "Robust XON/XOFF" is to blame ...

The robust Xon/Xoff should actually be called "robust Xon".  It's function
is to prevent loss of communication due to lost Xon's (Xon sent while the
host was already in a FIFO overflow situation).  The way it works is:

 * If the printer is not been busy, and
 * 5 seconds have elapsed since sending the last Xon,
 * Then send an Xon.

This can cause problems for hosts that fail to read and discard unexpected
Xons (Xons arriving with no prior Xoff).  Their input FIFOs can fill up with
Xons, resulting in unpredictable consequences.  I assume you have tried
running with "robust" disabled?


> Another problem is the inability to swap between PCL and PostScript
> without having to switch the printer off and remove the cartidge. I
> thought I was buying a machine that would run with both PCL and PostScript
> with some sensible method of software switching.

All the PostScript users inside HP agree with you on this, and I fully
expect future LaserJets to be more flexible in this regard.

Regards,                                              Hewlett-Packard
Bob Niland      Internet: rjn@hpfcrjn.FC.HP.COM       3404 East Harmony Road
                UUCP: [hplabs|hpfcse]!hpfcrjn!rjn     Ft Collins CO 80525-9599