OctaMED SS rev.2
Auto Flow Control
Up to our knowledge Twister (Twister1200 or on top of Unity) is the only serial interface board for Amiga which offers AFC besides the traditional mechanisms. The following text will try to explain why this is very important.
Some History and Background Information:
In former times the Amiga was known to be a Computer which took a majority of its performance not from the main CPU but from its Chipset. The chipset offered Coprocessors for sound, graphics and more. Unfortunately the engineers did not design a serial interface having equivalent strength because of price. While the chipset supplies the data, an additional interface chip (CIA) is used for handshake. The chip was done for the predecessor of the 68000 CPU and nearly the same device can be found inside the legendary C64. When used at Amiga the 68000 CPU has to slow down to 10% of it's native access speed.
At the early years of Amiga the lack was not noticeable because of very slow modems. Allthough people clearly noticed the trouble at faster Null-modem connections. As said before the chipset offers the data while an external unit has to handle the handshake. It is telling the communication partner when to go and when to wait. When having a closer look to the mechanism the limitation might get clear. The chipset is able to temporary store one character a time for sending direction an the same for receiving direction. If the interface would not be cut into two pieces the chipset could offer and get the characters by using a simple hardware based algorithm. Unfortunately there are some lines of code (so-called Interrup-Handler) at the AmigaOS which are controling the cooperative work of the chips. Beside handling the data flow in both directions it has to sign a temporary stall to the machine on the other end of the line. This is the mayor reason for following kind of trouble:
Having this in mind it's allready allmost unimportant how high the possible datarates on a serial interface could be. Real live throughput anyway is only a fraction of the physical maximum. The line drivers which are used at Amiga are not able to communicate faster than 115,200 BPS without violating the EIA RS-232 standard.
What do today's interface modules offer?
For the Amiga there are several serial interface cards. They try to fix the described probems with different effectivity. All newer extensions make use of the 16Cx50 family of chips. Therefore it's not too difficult do a comparsion. A higher number for the "x" means more enhanced capabilities. Well known chips are 450, 550, 650. The 16450 is called generation one in the following text.
Which features does the interface board offer?
We stopped trying to make fair comparsions to other boards. Keep in mind Twister is the only one for Amiga which really offers AFC additionaly to the old de-facto polling approch. It has got a driver which really makes use of the AFC capability if you like. Additionally it makes use of a powerfull line driver which usually is only used at much more expensive boards and typically drives 691,200 BPS (460,800 min.) safely. Since a generic user does not know about autoflow and wouldn't honor a higher price the 750 and 850 boards will probably never appear in series boards (increases price about USD 5). As said before your modem might prevent Twister from doing usefull transfer speed. If you can't fix it by increasing the BPS rate, you might better disable the AFC feature with the tool which comes with the board. You still make use of the bigger buffers compared to other boards. You might run into trouble when using a soundboard the same time, though. At least it works much better than the internal serial.
A look at other computer systems:
Don't expect modern machines are using nice chips on their boards. Some manufacturers tried to do so, but nobody payed attention (the guys from Seatle included). People simply don't care too much for a better chipset if it's more expensive. On the other hand there are expansion cards available which are strongly used by professional users (mostly Unix machines). They offer low system load even when a number of ports is running in parallel. The Mac made use of a really nice DMA driven Chipset from 1991 to 1998. Today it's replaced by USB. Bitrates of 230,400 BPS up to 1,000,000 BPS at strong CPS rates caused very low load, too.