Date: 2000-09-20 22:01:23

Hallo allemaal,

For connecting a PC with a IEC-device we have the X1541-cable. There is no
standard for connecting a PC DIRECTLY to the IEEE-connector, at least AFAIK.
So I am willing to propose a standard.

But there are some problems. The IEEE-interface has 8 bidirectional datalines
plus 8 bidirectional controllines. A PS/2 (or bidirectional) LPT-port has 8
bidirectional datalines, 4 OC-outputs and 5 inputs. Other ports cannot be
used (AFAIK). 

Some research on C= equipment revealed:

Line         NRFD   NDAC   DAV    ATN    EOI    IFC    REN    SQR
             I  O | I  O | I  O | I  O | I  O | I  O | I  O | I  O |
Listener   |    X |    X | X    | X    | X    | X    |      |      |
Talker     | X    | X    |    X | X    |    X | X    |      |      |
Computer   | X  X | X  X | X  X | X  X | X  X |    X |    G | X    |

X marks line used as "I = Input" or "O = Output", G = ground 

A printer is a typical Listener, a drive is a Listener or a Talker.
As you can see, C= doesn't use the REN- and SQR-line.

This research lead to my first design (cable 1):

          25-D  IEEE
D0          2     1    D1
D1          3     2    D2
D2          4     3    D3
D3          5     4    D4
D4          6    13    D5
D5          7    14    D6
D6          8    15    D7
D7          9    16    D8
STROBE      1     5    EOI              (I/O)
SELECT IN  17     6    DAV              (I/O)
INITIALIZE 16     7    NRFD             (I/O)
AUTO FEED  14     8    NDAC             (I/O)
BUSY       11     9    IFC (RESET)      (IN)
ERROR      15    10    SRQ              (IN)
ACKNWLDG   10    11    ATN              (IN)
SELECT     13    17    REN              (IN)
GND      18/25  19/24  GND

With this cable a PC can be used as drive and/or printer. 
Advantage:    simple
Disadvantage: PC cannot be used as Computer

But I had a higher goal and that was to be able to use Star Commander as well
for any IEEE-drive. This means we need at least a bidirectional ATN-line. I
had a discussion with Joe Foster and Nicolas and we had three options:
1) using an extra LPT-port
2) soldering extra IC's on a LPT-card
3) using a small interface plus a COM-port
      About the interface: IMHO you only need a diode, two resistors and a
                                      +----> ATN-line
      RTS ----- DIODE --- 3K3 --+-- B
                                |     E
                               15K    |
                                |     |

      I have seen interfaces using a FET (BS170 ??) because of the speed but
      I'm not familiar using FETs as OC-output. I don't think speed is a
      problem here. We can even use SW to check if the output has done what
      we told it to do. But if somebody is familiar with FETs, would he be so
      kind to tell the (dis)advantage of using a FET and how to connect it

The third option was the most flexible one and therefor choosen (cable 2).
Advantage:    fully compatible (AFAIK)
Disadvantage: needs an extra piece of hardware, connector and RS232-port

Testing my program PC-Disk using cable 1 I found out (so far) that not using
NRFD did not have any impact on the communication. "Not Ready For Data" can
be considered as an equivalent of the RDY-input of the 6502. When (L) the
sender halts the communication until it is (H). "No Data ACcepted" can be
used in this way as well but here I found out there are limits: I could delay
the communicating up to about 40 ms, using more delay caused the computer to
respond with an error. IMHO 40 ms are more then enough to do the job, even
when using a 4.77 MHz PC. 
Swap NRFD and ATN and we have cable 3. 
Advantage:    simple
Disadvantage: PC cannot activate NRFD  (may cause incompatibility ???)

But maybe you see "bears on the road" I overlooked so please think about it
and give me your opinion. 

Groetjes, Ruud


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