| Sign In Sign-Up

Welcome!
Close
Would you like to make this site your homepage? It's fast and easy...
Yes, Please make this my home page!
No Thanks
Don't show this to me again.
Close

## Ethernet AUI to AUI cheaply

    I **DID NOT** try out this wiring diagram, as the need evaporated before I had
    a chance to try it out.  (I found some BNC transceivers laying around.  :-)

    I've done the following, and got it to work. No promises, try at your
    own risk!

                    Shield   1              1  Shield
        Collision Detect A   2              2  Collision Detect A
           Transmit Data A   3 ----\ /----  3  Transmit Data A
      (Receive Data Shield)  4      X       4 (Receive Data Shield)
            Receive Data A   5 ----/ \----  5  Receive Data A
                        Vc   6 ---     ---  6  Vc
            (Control Out A)  7    |   |     7 (Control Out A)
       (Control Out Shield)  8    |   |     8 (Control Out Shield)
        Collision Detect B   9 ---     ---  9  Collision Detect B
           Transmit Data B  10 ----\ /---- 10  Transmit Data B
     (Transmit Data Shield) 11      X      11 (Transmit Data Shield)
            Receive Data B  12 ----/ \---- 12  Receive Data B
                        V+  13             13  V+
                 (V Shield) 14             14 (V Shield)
            (Control Out B) 15             15 (Control Out B)

    The AUI interface has a female host plugging into a male xceiver.  the host
    drives a transmit pair.  the xcvr drives a receive pair and a collision pair.
    if you want to plug a host into another host, you have nobody driving the
    collision pair for either host.  there is no logic in the host for treating
    as an error that condition called "transmit and receive both active" since a
    xcvr can't DO that.  instead the error in the host is "transmit and collision
    both active."  if a host sees that error it will fall back and retransmit i
    the hardware and sometimes in the driver.  without that error, packets sent
    while the other side is also sending will be silently dropped, and the higher
    level protocol (tcp, nfs, whatever) will resend after an ACK timeout or
    whatever.  this results in horrible performance.

    in a reasonable(*) ethernet host adaptor, transmit and receive are completely
    decoupled so there is no shared hardware and no reason why you can't send and
    receive at the same time.  (*) by this i mean one made by ungermann bass or
    excelan or xerox's research labs, or 3com's 3 megabit multibus cards.  "old."
    in such an adaptor you don't need collision signal for a cross over cable since
    there is no collision -- both hosts can send and receive at the same time.  in
    the modern era this is called "full duplex ethernet" but there's no standard
    for it on aui or thinwire transceivers and nobody other than -- ta da! --
    digital ever implemented it for 10baseT (on the PCI DE500) and even that is
    not understood by any driver i've ever seen except digital unix and openvms.

    so what you need to do is OR together the crossed over transmit/receive pair
    and the crossed over receive/transmit pair, and use the output to drive an op
    amp which in turn drives the collision pair for both hosts.  this way any time
    both hosts are transmitting, both will experience a collision and each will
    wait a random exponential backoff time before retrying.  (you could latch the
    op-amp output whenever one host is transmitting, so that the other one is the
    only one that would experience a collision, but with digital's host adaptors
    this wouldn't help since the damage would happen to both sides before the
    collision signal could be acted upon by either side.  rant, rant, rant.)

    this OR'ing and op-amp'ing is what a DELNI does.  i have one that i just took
    out of service and the first $20 (estimated shipping cost) will get it away
    from me.  for an added bonus, i'll include a pile of 2M AUI cables and -- get
    this! -- a 10 inch AUI cable made for me back at DECWRL by david boggs, whose
    name is the second of three on the ethernet patent (he built the first xcvr.)

    you can also get what you want with a pair of AUI/10BaseT transceivers, with
    a null-ether RJ48 cable (swap pairs 2 and 3, or wire pairs 1/2 and 3/6).  but
    if you want to stick with AUI, you need that OR and the op-amp, which means you
    have to make it yourself (noting that the transmit pair has DC power on it so
    you can run your midspan chips that way) or you have to use a DELNI.