From: Marko Mäkelä (marko.makela_at_hut.fi)
Date: 2003-06-07 09:36:34
Hello Ruud, > > 1. Can multiple bytes be sent with ATN asserted? > > Yes. The actual cammandcodes are sent with ATN = (L). I spent some hours last night, disassembling and commenting the serial bus routines of the VIC-20 KERNAL ROM. It looks like there can be one or two bytes sent under ATN: the UNTALK ($5f) or UNLISTEN ($3f) codes, or the device number 0..30 ORed with a command code (TALK=$40, LISTEN=$20), and the optional secondary address, optionally ORed with $f0 (for open) or $e0 (for close). If the secondary address specified with the SETLFS call is above 128, no secondary address will be sent. This was new to me; I only knew that file numbers above 128 are treated specially by CHROUT (it'll convert CR to CR+LF). > > 2. Can the talker and listener change roles when ATN is > > initially deasserted? > > No. The VIC-20 firmware agrees with you: the routine at $eed3, which changes from a talker to a listener, is entered after the TALK command (which can be one or two bytes). Also, it looks like that the only place where the VIC-20 generates EOI handshake is when sending the second byte under ATN. I'm not fully sure about this, and I haven't looked into drive firmware yet, to see if it tolerates EOI handshaking when receiving non-ATN bytes. > Quite simple if two drives are concerned: > - you tell the TALKER what file to send  > - you tell the LISTENER what file to receive  > - you tell the LISTENER to start to LISTEN > - you tell the TALKER to start to talk  Thanks, this makes sense. I analysed how the higher-level routines access the serial bus: open: - if secondary address is omitted (above 128), do nothing - otherwise, - send LISTEN, SECOND or $f0 under ATN - send filename with CIOUT calls - send UNLISTEN under ATN close: - if secondary address is omitted (above 128), do nothing - otherwise, - send LISTEN, SECOND or $e0 under ATN chkout: - send LISTEN and optionally SECOND under ATN chrout: - send the character with CIOUT chkin: - send TALK and optionally the secondary address under ATN - become listener chrin: - receive a character with ACPTR save: - open the file (cf. "open"): - send LISTEN, secondary address or $f0 under ATN - send the filename with CIOUT calls - send UNLISTEN under ATN - write the file (cf. "chkout", "chrout"): - send LISTEN, secondary address under ATN - send the data with CIOUT calls - send UNLISTEN under ATN - close the file (cf. "close"): - send LISTEN, secondary address or $e0 under ATN - send UNLISTEN under ATN load: - open the file (see "save") - read the file (cf. "chkin", "chrin"): - send TALK, secondary address under ATN, become listener - get the data with ACPTR calls - send UNTALK under ATN - close the file (see "save") Marko Message was sent through the cbm-hackers mailing list
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