To connect Linux's AX.25 to a Hexipus, use nrsdrv -f (enabling flow control),
and a serial cable as shown:

Pin numbers are for 9 or (25) pin connectors.

            TXD  RXD  GND  RTS  CTS                 DTR
Linux PC:   3(2) 2(3) 5(7) 7(4) 8(5) -- 4.7K Ohm -- 4(20)
             |    |    |    |    |      resistor
Hexipus:    2(3) 3(2) 5(7) 8(5) 7(4) 
            RXD  TXD  GND  CTS  RTS

The following is a more thorough explanation, extracted from the JNOS
readme.now file, and originally written by Dave Brown N2RJT:

 The hexipus and other diode matrixes for connecting NetROM-style nodes
 have always been a source of confusion.  This is an explanation of how
 and why it works.

 The hexipus was designed to connect two Tiny-2 TNCs with straight-thru
 cables. The Tiny-2 was designed to connect to a PC (9-pin connector)
 with a straight- thru cable, just like a modem.  That means the Tiny-2
 is DCE:  it receives data on the TXD pin, and sends data on the RXD pin
 - exactly the opposite of what seems right.

 The hexipus is wired like a null-modem, except with diodes.  That much
 makes sense - but you have to understand that the hexipus isn't a
 null-modem.  It is the opposite:  a null-terminal, null-computer, or
 null-DTE.  In other words, the diode orientations are exactly the
 opposite of what they would be if the hexipus were a null-modem.

 The other important thing to understand is that the data signals go thru
 the diodes in a different direction from the flow control signals.  This
 is easiest to explain in terms of voltage levels and idle states.  The
 purpose of the diodes is to allow a single line with non-idle state to
 override all the other lines with idle states.

 Data lines: the idle state is a STOP  bit = 1 = MARK  = V-
             the data state is a START bit = 0 = SPACE = V+
             One V+ must be able to override multiple V-

 Flow lines: the idle state is V+
             the flow state is V-
             One V- must be able to override multiple V+

 So, this is what the matrix must look like:
 -----------------------------------------------------+
           -              |           -               |  Symbols:
 Idle RXD --->|--  ~      | Idle RXD --->|--  +       |
           -     |--- TXD |           +     |--- TXD  |     + Positive Voltage
 Idle RXD --->|--         | Send RXD --->|--          |     - Negative Voltage
 -----------------------------------------------------+     ~ No current flow
           +              |           +               |
 Idle CTS ---|<--  ~      | Idle CTS ---|<--  -       |     All signals flow
           +     |--- RTS |           -     |--- RTS  |     from left to right.
 Idle CTS ---|<--       ^ | Send CTS ---|<--          |
                  pullup|                             |     Resistor = 4.7K ohms
                  resistor to V+                      |
 -----------------------------------------------------+

 So the simplified 2-port diode matrix would look like this (9 pin sense):
 2 RXD-->|--TXD 3
 3 TXD--|<--RXD 2    The RXD diode arrow always points to TXD
 5 GND------GND 5
 7 RTS-->|--CTS 8    The RTS diode arrow always points to CTS
 8 CTS--|<--RTS 7

 The more common cables are shown below, assuming 9-pin connectors, but
 showing the 25-pin connector assignments in parentheses.  Tiny-2s have
 the pullup resistor internally, but PCs don't.  The usual way around
 this is to solder the resistor to an unused pin such as DTR, and have
 the PC software assert DTR to supply the V+.

 LINUX OR
 PC NOS       HEXIPUS | PC MSYS      HEXIPUS | TNC-2        HEXIPUS | Tiny-2
 ==================== | ==================== | ==================== | ==========
 TXD 3(2) -- 2(3) RXD | TXD 3(2) -- 2(3) RXD | RXD 2(3) -- 2(3) RXD | RXD 2
 RXD 2(3) -- 3(2) TXD | RXD 2(3) -- 3(2) TXD | TXD 3(2) -- 3(2) TXD | TXD 3
 GND 5(7) -- 5(7) GND | GND 5(7) -- 5(7) GND | GND 5(7) -- 5(7) GND | GND 5
 RTS 7(4) -- 8(5) CTS | DTR 4(20)-- 8(5) CTS | DTR 4(20)-- 7(4) RTS | RTS 7
 CTS 8(5) -- 7(4) RTS | CTS 8(5) -- 7(4) RTS | CTS 8(5) -- 8(5) CTS | CTS 8
 DTR 4(20) asserted   | RTS 7(4) asserted    |                      | NR  9
                      |                      |                      |
 Solder resistor at PC| Solder resistor at PC| Connect pins 10 & 23 | Connected
 end of cable to CTS  | end of cable to CTS  | at TNC-2 end of cable| straight
 and DTR which is V+. | and RTS which is V+. | to enable NetROM mode| through.

--
Dave Brown N2RJT <dcb@vectorbd.com>
