AppendixCapNet & Deskwriter
Appendix
How to use CapNet with the Hewlett-Packard DeskWriter (AppleTalk version only)
The first release of the HP DeskWriter did not support AppleTalk. These comments apply only to the later versions, including the HP DeskWriter C.
The DeskWriter supports both a direct serial connection and an AppleTalk connection. It decides automatically which communication method to use. Unfortunately, this means that it will not work with standard CapNet as described above. The DW measures the resistance between the TXD+ and TXD- lines. A LocalTalk connector box has a low-resistance isolation transformer connected across these lines, whereas a serial connection appears as a high resistance. Standard CapNet has a 2 KOhm resistance across TXD+ and TXD-, which is high enough to fool the DW into thinking it's talking to a serial port. It therefore uses the direct connect 56 Kbaud protocol, and the AppleTalk packets it receives from the Mac appear as garbage on the page.
The solution is to connect an additional resistor between TXD+ and TXD-. I don't know what the exact value should be, but a 1 KOhm resistor works for me. If you find it doesn't, try a 500 Ohm resistor, that should be sufficient in any case.
Here is the revised schematic
PARTS LIST
| QTY | DES | ITEM |
| 3 | R1,2,5 | 1K 1/4W RESISTORS |
| 2 | R3,4 | 10 OHM 1/4W RESISTORS |
| 2 | C1,2 | .1 uF THREE LAYER CERAMIC CAPS |
| 1 | J1 | DB-9 MALE OR MINI DIN 8 CONNECTOR |
| 1 | J2 | 2 PIN BERG OR RJ-11 PHONE CONNECTOR |
| 5 | WIRE | (NOT SHOWN) 22 GAUGE STRANDED WIRE 6" |
Disclaimer
The described modification of CapNet has received only limited testing. I have tried it with two DeskWriters, a Mac II, an SE/30 and a PowerBook 140, and it seems to work. However, use only at your own risk. I specifically disclaim all responsibility for any problems that may result from its use. Share and enjoy.