T

ROM 3 ISOOOH-3FFFM)

ROM 2 (2000H-2FFFH)

ROM I (IOOOH-IFFFH)

ROMO (OOOOH-OFFFH)

TO ROM CHIP SELECT LINES (PIN 18)

Figure a. Changes to printer buffer circuit for additional ROM space. All four ROM sockets are wired the same as the ROM socket shown in Fig. 5c of the original schematics, except that they each get different chip select signals, and the pin 21s can all be tied to A1 IB without a jumper.

Below are listed changes necessary to complete the printer buffer in the September 1984 Project 80 (p. 102).

• You need five lk ohm resistors instead of the three indicated in the parts list. You need four if you don't use Ropt as described in the text.

• You don't need a 10k ohm resistor, though one is indicated in the parts list.

• R10 is the 4.7k ohm resistor used to pull up NMI/, BUSREQ/ and WAIT/ on the Z80 (Ul), although it isn't marked R10.

• The parts list mentions two quad two-input AND gates with the part number 74LS04. This should be two quad two-input NAND gates with the part number 74LS00.

• Some readers had trouble finding the U29 on the schematic. U29 is not a true IC, but merely a delay line, shown near the lower 74LS157 (U13) in Fig. 5d.

• A note about the 74LS51 gates (U31) in Fig. 5e: Each gate consists of two AND gates and one NOR

gate. The combination of these, all on the same chip (actually two per chip), is called an AND-OR-INVERT gate.

• I have had some requests for copies of my original schematics if the ones redrawn in 80 Micro were in error. As far as I can tell, the schematics are reproduced identically.

• Some readers have asked about the additional circuitry required for the extra ROM space I mentioned in the column. If you need more than the 4K offered by the 2723A, you can add the circuitry shown on the schematic diagram (Figure a). This circuitry gives you up to four 2732As in your system, with up to 16K of ROM space. Note that the circuitry is designed only for 2732As, not for 2716s. AS of the ROM sockets should be wired the same as the ROM socket shown in Fig. 5c of the original schematics, except that you should connect all of the pin 21s to A1 IB. This way, each socket gets its own chip select signal (pin 18). ■

GENEALOGY and INDEXING software

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system and user's manual: $128.45

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TRS-80 Models 4/4P or 11/12/16 TRSDOS 6.x/ 2.0a/ 4.2 system and user's manual: $195.00

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TRS-80 Models 4/4P or 11/12/16 TRSDOS 6.x/ 2.0a/ 4.2 system and user's manual: $99.00

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