Allison wrote: last time I checked the Zilog 8530 didn't have any dma. it's not capable of being a bus master but itself. perhaps we're talking about different things? -brad

Allison wrote: That chip sucked when used as a plain old UART. In order to handle slight speed mismatches between two modems emplying synchronous modulation, without requiring the modems to have flow control to the hosts, there's an ITU-T (formerly CCITT) spec that allows stop bit shaving. Characters output by the modem to the host is allowed to have a slightly short stop bit (15/16 of a full bit time). This would happen if one modem had a slightly higher speed crystal than the other; that modem could send bytes every so slightly faster than the receiving end could put them out its serial line. This was common with 1200 bps through 9600 bps modems before MNP and V.42 error control were used; when error control is used there is flow control between modems, and flow control required between hosts and modems, so stop bit shaving is not needed. Some other non-modem devices generate fractional (shaved) stop bits as well, for similar reasons. Normal UARTs, SCCs, etc. handle fractional stop bits just fine, because they only sample the stop bit near its midpoint, and are reading for another start bit soon after. The uPD7201, on the other hand, will reject the shaved character and/or the following character with a framing error. There's another related modem feature used by modems with synchronous modulation and no error control. When the host serial port sends data to the modem slightly faster than the modem's synchronous modulation (again, due to crystal variations), the modem drops up to one in ten stop bits entirely when running them through the modulator. (On the synchronous modulation, it's not possible to send a fractional stop bit). The receiving modem knows to deal with this. This condition tends to require the use of stop bit shaving on the serial port, since host computers will not accept characters with entirely missing stop bits. The stop bit deletion on the modulation is part of the ITU-T V.14 standard, and is allowed on no more than one character in ten, so it allows the modem to deal with up to 1% overspeed. The recieving modem may or may not have to employ fractional stop bits on its host interface to compensate. Eric