Hello,
My two cents worth ... I have been thinking of this before but I don't have
the time and knowledge to develop computer boards. (Ps. I'm not a expert on
the matter but I feel I have a fair understanding of the technology.) See
remarks in the comments. I think you should consider why you want to build
such a card. Because there are already numerous cards out there you need
something extra to differentiate.
About bus architectures:
ISA old standard but still many 386/486 and older P5 systems
with ISA busses in use (I have two of them)
PCI the new standard, more I/O, IRQ and DMA space
USB not sure if DMA is possibly but might be usefull to consider
About packet speeds:
The current cards perform quite well with 1k2, 9k6 and even 19k2 modems.
However, if you have a large(r) node with say two SCC board (is 8 channels)
you might run out of processing power. I'm not sure how much data a 82C230
can buffer but it not more that a few bytes. Consider the number of
interrupts on a system with a lot of busy 19k2 interlinks.
The BayCom EPP adapter (http://www.baycom.org) reduces the number of
interrupts by utilizing the 8-bits wide parallel poort and an external
parallel to serial (the EPP adaptor) to drive 76k8 modems. This technique
reduces the interrupts by at most the factor of eight.
Next step is DMA. With DMA data is delivered into the computers RAM for
immediate use. No interrupts (unless maybe a "packet ready" signal) and data
polling by the driver sparing a lot of CPU cycles. In my opion the way to go
for speeds beyond 38k or so. What lacks is SCC boards supporting DMA (at
least I don't know of them).
The ideal card:
- four channels with programmable bit rates up of 1200 to 2 Mbit/sec (split
rates for RX/TX possible).
- full duplex
- DMA support
- ISA and PCI version, maybe USB
Regards,
Feico
-----Original Message-----
From: Jason Flynn [mailto:[EMAIL PROTECTED]]
Sent: Thursday, June 03, 1999 18:22
To: [EMAIL PROTECTED]
Subject: New SCC card design
I've been looking into designing a new SCC card for packet
and making the PCB masks freely available on the 'net.
I've currently got a USCC and a S5SCC card and I've had
a good look at how each of them work. I feel that a PCI
based card is beyond my capabilities so I'm talking ISA!
I came up with a few points I wanted to discuss with people:
1) What IC would be a good move 85C230 or 16C3x or
something different again ?
Remark:
The advantage of the 16Cx (couldn't find the 16C3x on the Microchip site) is
that some of them have a build in serial interface. I'm not sure if they can
do HDLC encoding or if it are just synchronous serial lines. I know Motorola
has controllers (I think in the 68k series) with embedded HDLC stuff) that
might be more usefull despite the probably higher price. If you use the
85C230 chips you probably want to add a microcontroller anyway to do some on
board processing.
2) What is a good way of getting the data of the SCC IC?
ISA DMA? DMA into on card memory mapped RAM ?
Remark:
I don't think the latter can be considered DMA but rather PIO. PIO is en
general slower because some driver has to copy from the card memory into
kernel or other memory for further processing while with true DMA the data
already is in place.
3) What are the issues with running both channels
of a Dual channel IC in DMA mode?
Remark:
This might by beyond my knowledge. Likely you need more the one (read two)
DMA channels. If you have on card memory you might consider some fancy
buffering and a single DMA channel but this will increase driver complexity.
As usual it is a tradeoff.
4) What are the issues with running full duplex
and DMA ?
Remark:
See previous remark.
73 Jason
PS can anyone explain exactly what the bug in the
z8530 is that everone complains about ?
PPS can anyone point me to an online manual for
the 85C230?
Remark:
http://www.intel.com and use the search option.
