SUMMARY: Increasing Suns Serial Port Speed

From: Stephen Pendleton (
Date: Mon Sep 04 1995 - 16:24:42 CDT

Previously I asked:

I have attached a Multitech 28.8 modem to my Sparc 10's serial port and
it is currently running at 38400 bps. I am interested in driving the
port speed up to 115200 to take full advantage of the 28.8 modem. Is
this possible using the standard Sun serial port? Do I need to modify
the kernel to support speeds above 38400?


I received several answers. To summarize: the answer is yes it can
be done but it is very unstable to do so. Get a SBUS I/O card.

In more detail here are some of the responses:

From: Syed Zaeem Hosain <>
The standard serial port speed is limited to 38400 - the few known
kernel patches allow 56000 and 76800. The first of these is useless for
any known convenient serial device, and the second is problematic due
to the lack of proper buffers in the Sun serial port. Plus. most modems
(ZyXEL is an exception) do not use 76800.

I recommend putting in an add-on SBUS serial card. For example, from
Magma Systems (in San Diego, CA). This will get you to 115200.

From: Kevin Sheehan {Consulting Poster Child} <>
Yes - 38400 is the highest the driver knows about. To go faster you
either need to get the high speed card, or you need to patch the
zs_speeds array in the kernel.

                l & h,
From: Gary Merinstein <>
no, you need to modify the ss10 hardware; the ss10 serial port "canna
handle the strain!" (sorry I'm watching ST..). Sun sells a high-speed
serial port add-on, as does some other companies (I use an aurora card to
get higher serial i/o).

From: "Erik J. Tolido" <>

I believe you'll need to install Suns HSI S-Bus card which allows for
high speed serial communication over, for instance, 1.544 Mbps T1 links.
The only drawback that I found, is that is uses RS-449 connectors.

Erik Tolido

From: Celeste Stokely <>
It can be done, but it's highly unstable and you have no useful flow
control. If you need to use higher speeds, buy
a DMA-based SBUS serial port card with true hardware flow control.

..Celeste Stokely, Unix System Administration Consultant, Stokely Consulting

From: Jeff Wasilko <>
The system is not reliable at speeds above 38400.

I'd reccomend you look at an external serial port (Either SBus or
scsi-based). Central Data ( has a fairly cheap 2 port
scsi-terminal server that is very nice. It goes to 115k with full
flow control.

From: Lisa Barnes <lisa@magma.COM>
Hello Steve:

Native ports on a Sun can handle up to 38400 baud in async mode but
typically have a problem with high-speed serial devices where high speed is
defined as 19200bps or faster. Magma's SBus serial cards overcome this
problem with hardware flow control and will provide you with the speed your
looking for (115.2Kbps) without overloading the CPU.

Any of our serial port products will meet your requirements. For your
review I have attached a summary of our SBus products and data sheets on our
LC 2+1 Sp and 2+1 Sp (both products provide 2 serial ports and one parallel
port). The difference between the LC 2+1 Sp and our standard 2+1 Sp is the
speed of the parallel port, so if you are only interested in the 2 serial
ports the lower cost version may be the best choice for you.

From: raoul@MIT.EDU
Don't bother. The advantage of higher serial speeds on a PC modem
is based on their interrupt driven serial controller: when the
PC gets around to the PC handling software, it accepts the
interrupt and downloads a buffer at the 115200 speed and
goes on to its next task, downloading on the next go-round
only if there is space for it.

UNIX does proper multi-tasking, letting the interrupt of the serial
controller be handled in a timely fashion even if it interrupts other

Now, for hardware flow control, or good serial drivers, you may
need a patch. What OS do you use?

Thanks to all!!!


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