From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Platt Subject: Re: YAM modems at higher (shared) IRQ's Date: Fri, 11 Mar 2011 13:36:46 -0800 Message-ID: <4D7A95EE.9060206@radagast.org> References: <4D73F262.6000703@exemail.com.au> <4D74B845.4040600@bfs.de> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-hams-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: linux-hams@vger.kernel.org On 03/09/2011 07:07 PM, Curt, WE7U wrote: > On Wed, 9 Mar 2011, Curt, WE7U wrote: > >> What I found out: The 16550 interface that I was trying to get >> running... ain't. It's a 16550-compatible multi-port card, NOT a card >> with 16550A's. > >> Worse, I examined the 2-port cards I have: They're also >> 16550-compatibles (SYBA 2-port PCI cards). > >> In my spare parts I found one PCI card with a real 16550A, a >> single-port card. I wonder... how much of the compatibility problems you're seeing, have anything to do with the chip itself (16550 vs. "compatible") and how much have to do with the external analog interfaces used by the board vendors? A lot of serial ports these days are only "barely RS-232", and some don't even comply with the RS-232 spec. RS-232 requires both positive and negative voltage swings (minimum of +/- 3 volts if I recall correctly, with +/- 12 having been the old "nominal" swing). Modern serial ports may only swing +5 and 0 (no negative swing at all), or may swing only just past the +/- 3 volt limits. The receiver thresholds on such devices are often set at around 1.5 volts or so. Furthermore, the amount of power which can be "robbed" from the handshaking lines is often quite miniscule... just a few milliamperes. Since the YAM is intended to be port-powered, I suspect it is going to be very prone to not work correctly when used from these wimpy, under-voltaged-and-under-powered ports. If the handshaking lines can't pull well above +5 while providing enough juice to run the FPGA, the device probably will not initialize properly. Here's my guess: - If you have a serial port with a 1488 quad-line driver IC, it's likely to work, as these drivers are powered from +12/-12 rails and have plenty of current drive capacity. - If you have a serial port with a MAX232 or similar voltage-doubling driver, it may work, depending on the actual output voltage generated on the handshaking pins and the amount of current which can be drawn. Voltage-doubler chips which have small on-chip capacitors may be less likely to have the necessary current-sourcing capacity, than those which use larger (e.g. 1-to-10 uF) off-chip caps. - If you have a serial port which simply provides "pseudo-RS-232" driver (inverts the TTL/CMOS logic, with no negative voltage swing) I suspect it's unlikely to work. As I see just now that Ray has suggested, you may be able to overcome the RS-232 voltage issues by using a separate (outboard) power supply to run the YAM circuitry. This might provide better compatibility with a broad range of modern serial ports, which are usually very register-compatible with 16550A UARTs but which differ at the electrical level.