Per our discussion. Alex ----------------------------------------------------- MECHANICAL A-B SWITCH BOXES AND THE HP LASERJET by Jim Baker and Emerald Jones (UCLA PC Users Group) Oops! Your Hewlett-Packard LaserJet breaks. If a specific set of chips has been fried (the input circuitry), you are stuck with a big repair bill. Hewlett-Packard will *not* honor a warranty if your Laser-Jet printer has been operated from an A-B mechanical switch box. That's because the mechanical switch boxes have a habit of frying some circuits, and besides, the Hewlett-Packard documentation contains a caveat warning people to use only electronic switch boxes with these printers. Hewlett-Packard recommends you use an electronic switch box, which, because of its internal wiring, doesn't endanger Hewlett-Packard's input circuitry. The threat to input circuitry on any peripheral exists when a mechanical box is used to switch between the peripherals. Hewlett-Packards are known to be sensitive to this condition. Comparable difficulties have been reported on Okidata printers and surely affect others. Note that the equipment is at risk regardless of whether the connectors are serial or parallel, 25-pin or 36-pin, regardless of which type of mechanical switch box you are using. Scenario: Two printers are attached to one computer through an A-B mechanical switch box. One is a Hewlett-Packard LaserJet, and the other is a conventional dot matrix. The operator works on the computer, using the switch box to change between one printer and the other. All equipment is turned on all the time. When either printer is used, the signal (or logic) ground to the other is disconnected. In other words, when the dot matrix printer is being used, the signal ground between the LaserJet printer and the A-B box is interrupted. Since it has its own power source, the LaserJet's internal logic ground level settles to a slightly different level than when it is connected through the box. When the box is switched to connect the LaserJet printer, if the power happens to go to the signal pin first, the LaserJet input chips and circuitry must absorb the energy required to equalize the levels. These chips cannot handle the transient energy and are stressed. The damage is cumulative and may eventually destroy the chips. Whether the power goes to the signal pin first or the ground is a function of the switch box. Usually it is random. To determine if there is a power-switching hierarchy requires looking at the power switching with an oscilloscope. Not a realistic choice for many people. Note that turning off the LaserJet printer when it's not being used, and not changing the A-B switch box position when it is plugged in will not necessarily isolate the printer from all surges. This is because EMI filters on the computer connected to the printer can bleed into the printer's input circuitry. It is possible for the computer ground to pass voltage from the computer (on) to the printer (off). To eliminate the hazard, make both the signal ground and the shield (chassis) ground contiguous between the printer connectors to the computer connector within the switch box, regardless of which unit is operating. 1. Connect a wire in the box to make the signal ground circuitry contiguous from connector to connector within the switch box. 2. Also make sure that the shield ground (carried by the cables) is good from one printer through the box to the other printer. Tighten all screws connecting the cables and check continuity with an ohmmeter. Another option: Use an electronic A-B switch box. For design reasons, the logic ground in these boxes remains intact regardless of which printer is on-line. The "newer cheap ones" are about $100. The following describes the pins to be wired, depending on the connector type. It involves soldering between the pins within the box. ***Do not attempt to do this unless you have some experience soldering electronic components.*** While it's fairly simple, it should not be attempted by the novice. For one thing, should you do it incorrectly, you jeopardize some expensive equipment. While every effort has been made to clearly describe this procedure, if you attempt it and damage your equipment, the authors of this piece cannot be responsible. Mechanical boxes come in two different types: those with 25-pin "DB" connectors (either parallel or serial) and those with 36-pin Centronics (parallel only) connectors. In effect a "Y" structure, the box has a connector for the computer and, in our A-B example, two connectors for the printers. Solder a minimum length of insulated wire from any one of the pins on the computer connector to the corresponding pins on the printer connectors, I.e., pin 19 to pin 19. On the serial connector, all pins to pin 7. Then check your work with an ohmmeter. This will ensure that the signal ground is contiguous, regardless of which unit is connected. Appropriate Pins: any one of the pins noted is appropriate to use with standard cables. 25-Pin Parallel wiring: Signal ground pins = 18 through 25; use only one of the eight pins available. 25-Pin Serial wiring: The signal ground pin (ANSI standard) is pin 7; the shield ground is pin 1. 36-Pin Centronics parallel wiring: Signal ground pins = 19 through 30; use only one of the 12 pins available. NOTE: Once you've hardwired the grounds on the boxes that have 25 pins, you have, in effect, converted the box to either a parallel or serial box. Once it is so wired, you may not use a box converted to parallel with a serial hook-up and vice versa. Label the box clearly, so that no one can use it incorrectly. (Jim Baker is proprietor of Computer Repair Center, Woodland Hills, CA, where he repairs computers and peripherals. Emerald Jones repairs writing from Newbury Park, CA.) [I found this article in the September and October 1988 issues of the Pasadena IBM Users Group newsletter "Prompt>," but it was apparently copied from the UCLA PC Users Group newsletter.]