Subject: RE: Setting up a T1 and email
To: None <port-vax@NetBSD.ORG>
From: John Wilson <>
List: port-vax
Date: 07/16/1998 00:45:12
>From: Dave McGuire <>

>  Which is exactly why, in the VAX/PDP world, the DH11 series serial muxes were
>designed.  On my 11/34a I had a DH11-AD, which was 16 serial ports with full
>modem control that DMA'd incoming characters into buffers in memory, not
>bothering the host CPU.  It was very, VERY nice...especially since it replaced
>an interrupt-per-character DZ11. :)

Actually, reception works pretty much identically between the DH11 and DZ11,
they both tag incoming characters with their line number and stick them into
a unified FIFO which the CPU sucks out on interrupts (the DH11 gets to set
the FIFO alarm level though).  Transmission is where the big difference is,
the DH11 can send a whole buffer via DMA with just one interrupt at the end,
and the DZ ints after each character goes out.

DMA input sounds like a great idea but it would add a lot of complexity, in
regular (cooked) mode the CPU will just grab each character right back out
of memory one at a time anyway to decide whether it does anything special,
and then on the rare occasions when you get to do full bore binary input it
will need basically a whole second version of the receive handler that just
packs up a buffer at a time at a time and ships it in.

John Wilson
D Bit