![\"\"](\"docs\/btx_decoder_modul\/bildschirmtext.jpg\")
<\/a><\/p>\nBildschirmtext (Btx) was a German online service that existed from 1983 to 2001. Here are some videos<\/a> of what it looked like.<\/p>\n To use the service with a C64, this decoder cartridge was required, which connected to the expansion port.<\/p>\n The cartridge is actually a Siemens product, it’s just Commodore-branded. The sticker says:<\/p>\n The box with the Deutsche Bundespost logo is the registration (A506 131V). At the time, only explicitly Post-approved devices and software were allowed to be used for participating in Bildschirmtext.<\/p>\n (This is the “Btx Decoder Modul II”, even though the cartridge doesn’t say so. There was a completely different first model.)<\/p>\n There are three DIN ports at the back, from left to right: The Post-approval for participating in Bildschirmtext required the decoder to be able to show a pixel-exact image of the service (480\u00d7240 pixels with 32 out of 4096 colors)1<\/a><\/sup>. Since the VIC-II in the C64 could not meet this, the decoder cartridge had its own video controller and therefore its own video ports.<\/p>\n This is the output of the cartridge on startup. Note the different font, with 12×10 pixel characters. (The VIC-II is still active and shows a message that the user should connect the video cable to the cartridge instead.)<\/p>\n The board reveals that the cartridge doesn’t only have its own video chip, it also has its own CPU, RAM and ROM \u2013 it’s a complete computer that only connects to the C64 in order to reuse the keyboard and the disk drive.<\/p>\n These are the major chips: The chipset and the firmware are probably very similar to the technology used in dedicated Bildschirmtext terminals.<\/p>\n A hardware decoder was only required until 1990, when Deutsche Bundespost was finally convinced to allow software decoders. It was then enough if the pixel-exact image could be printed<\/em> to paper, which was met as long as the decoder software included a 24-pin (black and white) printer driver.↩<\/a><\/p>\n<\/li>\n<\/ol>\n<\/div>\n","protected":false},"excerpt":{"rendered":" Here are some pictures of the outside and the inside of a Commodore Bildschirmtext II Decoder Cartridge for the C64 \u2013\u00a0a 6800-based computer connected to the expansion port! Bildschirmtext (Btx) was a German online service that existed from 1983 to 2001. Here are some videos of what it looked like. To use the service with … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7,41,8],"tags":[],"class_list":["post-1310","post","type-post","status-publish","format-standard","hentry","category-bildschirmtext","category-c64","category-commodore"],"_links":{"self":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/1310","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1310"}],"version-history":[{"count":1,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/1310\/revisions"}],"predecessor-version":[{"id":1859,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/1310\/revisions\/1859"}],"wp:attachment":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1310"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1310"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1310"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}<\/a><\/p>\n<\/a><\/p>\n\n
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\n* Modem: This connects to the proprietary Deutsche Bundespost “DBT-03” modem (1200 baud down, 75 baud up).
\n* RGB: This is for connecting an analog RGB monitor.
\n* FBAS: This port is identical to the C64 video port and allows using a C64 monitor.<\/p>\n<\/a><\/p>\n<\/a><\/p>\n
\n* Motorola MC6803: a 6800-family microcontroller
\n* Motorola MC68HC34: Dual-Port Memory Unit
\n* 64 KB RAM (two 41464 64Kx4 DRAM)
\n* 32 KB EPROM<\/a>
\n* D65040GF206: NEC \u00b5PD65000 series CMOS gate array<\/p>\nThe hardware hasn’t been analyzed much.<\/del> The communication with the modem, the decoding of the CEPT<\/a> data stream and the interaction with the Btx service are handled by 6800 CPU and the software in the EPROM.<\/p>\nI am assuming the software writes the RGB values of every single scanline into the dual-port memory in real-time, which the gate array then reads to generate the video signal \u2013 like on the Atari VCS and the Sinclair ZX81. This keeps the complexity of the video hardware minimal.<\/del> The firmware maintains a 40×24 cell buffer in DRAM (four parallel planes \u2014 character codes, attribute bytes, diacritics, boundary-edge flags) along with per-row attributes and cursor state. The gate array reads this DRAM to generate the video signal. Similarly to the 6510\/VIC-II on the C64, the gate array takes its DRAM cycles when the CPU isn’t using the bus. The CEPT palette is uploaded into the gate array’s I\/O registers by the firmware.<\/p>\n<\/a><\/p>\nReferences<\/h1>\n
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