Date: Mon, 1 Mar 93 22:24:55 EET From: WB2PSI @ WB2PSI.#WNY.NY.USA.NA Message-ID: <6206.sv1rd@sv1rd.ampr.org> Subject: How NetROM matrix works FROM @BBS DATE WB2PSI ALLUSA 930301 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+ Note: on most Breakout Boxes, GREEN=SPACE and RED=MARK 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 = 4.7 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 softare assert DTR to supply the V+. 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. this file \wiring\hexipus.how on wb2psi BBS 18 Feb 93 Jim WA2ZKD@WB2PSI.#WNY.NY.USA.NA E-Mail: jpll@troi.cc.rochester.edu Dave N2RJT@WB2PSI.#WNY.NY.USA