[[Image:Amstrad_Computer_User8512_006.jpg|thumb|400px|right|The Cirkit serial link was used for the Cirkit Prestel link]]
ReportedlyThe Prestel Link Interface is a serial interface from [[Cirkit]] and [[HoneySoft]], it did "flourish"the interface was bundled with a modem (or actually, but almost nothing else an acoustic coupler). The interface is knownmainly intended for being used with that modem. It can be also used as a general purpose RS232 interface, but there are several restrictions: The outputs are TTL levels (which won't work with devices that insist on proper RS232 voltages), only a few baudrates are supported, only RX, TX, RTS, DSR are implemented (no CTS,DTR).
* Some information fragments are here: [['''Note''' - Cirkit]].reportedly also made a "Twin-port" serial interface (which probably used other chipset)
== Technical ==
(information from the documentation with the "Prestel Link" software.)The interface uses only two I/O ports:
Prestel link hardware:* 4Mhz clock from expansion bus is clock for 8251* Chipset: uses Port FADFh - [[8251 USART chip|8251 UART]] * I/O ports are:Data register FADF read/write data FBDF read statusPort FBDFh - [[8251 USART chip|8251 UART]] Status/write controlControl register
For details on the I/O ports see [[8251 USART chip]]. For details on the interface see [[Media:Cirkit prestel link hardware.txt|hardware.txt]].
* Note - Cirkit reportedly also made The baudrates are derived from a "Twin-port" serial interface 614KHz oscillator, divided by 32 (=19.2KHz), the TX rate can be optionally further divided by 16 (=1200Hz) (this is done by setting the DTR bit, which probably used other chipsetis misused to latch the baudrate selection, rather than to output a DTR signal), the RX rate doesn't have such an extra divider. Moreover, that rates can be divided by 1, 16, or 64 (inside of the [[8251 USART chip]]. The officially supported RX/TX rates are 1200/1200, 300/300, and 1200/75 baud. Further undocumented rates are 19200/19200, 19200/1200, and 300/18.75 baud (though they may be too slow/fast to be of practical use).
== Pictures ==
