a bad day for vaxen

From: dave06a at dunfield.com To: cctalk at classiccmp.org Date: Sat, 23 Feb 2008 07:08:19 -0500 Subject: RE: a bad day for vaxen An essential tool for anyone interfacing to serial equipment is a "light box" - just a box with LEDs to show which signals are being driven by a particular interface. I prefer the kind with two-color LEDs which show the polarity of the signal. To determine if you need a null modem, you need to determine which signals are being driven - You can use a LED or a multimeter set to measure current (few milliamps), in series with a suitable resistor to limit the current (few 100 ohms). The PC is setup to look like a DTE (Data Terminal Equipment), which means it drives (25 pin connector numbering): TD - Pin 2 RTS - Pin 4 DTR - Pin 20 The device which interfaces to it must look like a DCE (Data Communications Equipment) and should drive: RD - Pin 3 CTS - Pin 5 DSR - Pin 6 DCD - Pin 8 (Not usually required unless the software is expecting a modem) Note that the PC 9-pin connector has a different pinout that the traditional DB-25. This little chart may help: [View this message with a fixed pitch font - if you can't easily do that, save it to a file and view it from a text editor running on a text mode (non-graphical) screen] PC (25 pin) AT (9 pin) 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 14 15 16 17 18 19 20 21 22 23 24 25 6 7 8 9 Signal name Origin 25-Pin(PC) 9-Pin(AT) -------------------------------------------------------- Protective Ground (GND) --- 1 - Transmit Data (TXD) DTE 2 3 Receive Data (RXD) DCE 3 2 Request To Send (RTS) DTE 4 7 Clear To Send (CTS) DCE 5 8 Data Set Ready (DSR) DCE 6 6 Signal Ground (GND) --- 7 5 Data Carrier Detect (DCD) DCE 8 1 Data Terminal Ready (DTR) DTE 20 4 Ring Indicator (RI ) DCE 22 9 Connectors viewed from rear of PC. With all your cabling attached to the VAX, check the current being provided on TD, RD, RTS, CTS, DTR and DSR - note that not all of these may be driven - it is common practice to "loop back" signals which are not available on the interface. For example, RTS may be connected to CTS - so that when the PC asserts RTS, it will see CTS - even though that signal is not provided by the terminating equipment. If your VAX cable is providing current on RD, and NOT on TD, RTS or DTR, then it should be directly connected to a PC. If the VAX cable is providing current on TD and NOT on RD, CTS or DSR then you will need a "null-modem" cable which swaps TD/RD, RTS/CTS and DTR/DSR. If your interface is providing current on a combination of signals which originate at both sources (example, TD and CTS) then you have a mis-wired cable. I've had good luck making MMJ connectors from RJ-45 Ethernet connectors with a dremel tool - if you are careful, the result fits nicely, and even locks into place correctly with the tab. I can post a couple of photos if you want to see. The adapter I created to interface the PC to my VAXen looks like this: [View this message with a fixed pitch font - if you can't easily do that, save it to a file and view it from a text editor running on a text mode (non-graphical) screen] 6 3 7 - 2 20 DB-25 serial pins (so chart above for names & DB-9) + | | | | | | + + | | | | | | + View of top side of connector + + Looking down on connection pins. + + +-------------+ | Cable | Since RTS/CTS are not provided on the VAX MMJ interface, my cable also loops RTS to CTS. This connects directly to the PC (no null modem). Just in case it's "been a while" ... Don't forget that some VAXen seem to take quite a few seconds before the serial output of the self-tests starts after power-on. The first time I powered up a VAX I thought it wasn't working at first... Dave -- dave06a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.classiccmp.org/dunfield/index.html