TRS-80 Model 4P

TRS-80 Model 4P

TRS-80 Model 4P with the keyboard in front of the unit (photo: David Elsener, CC BY 3.0)
Specifications
Manufacturer	Tandy Corporation, sold by Radio Shack
Type	Transportable computer
Released	15 November 1983
Discontinued	1985
Intro price	US$1,799 (1983)
CPU	Zilog Z80A, 2 or 4 MHz
Memory	64 KB RAM, expandable to 128 KB
Storage	Two internal single-sided 5.25-inch drives, 184 KB each
Display	Built-in 9-inch monochrome CRT; 80 x 24 or 64 x 16 text
Sound	—
ROM	4 KB boot ROM; Model III BASIC loaded from disk
Power	105-130 V AC 60 Hz (US); internal 65 W switching supply
Dimensions	9.3 x 16.5 x 13.25 in (H x W x D)
Weight	26 lb
OS / Firmware	TRSDOS 6
Model no.	26-1080; 26-1080A (gate array)

The TRS-80 Model 4P is the transportable version of the TRS-80 Model 4, made by Tandy Corporation and sold through Radio Shack from 15 November 1983 at a starting price of US$1,799. The computer, a 9-inch monitor, two floppy drives and the power supply share one case. The keyboard stows in a recess in the case, and a removable front cover protects the screen and drives in transit and serves as a base when the unit is in use. It runs Model III and Model 4 disk software.

History

Radio Shack advertised the 4P as a Model 4 that could be carried on to a plane or train. It cost $200 less than the desktop Model 4. The 1985 catalogue cut the price to $1,299, and on 22 April 1985 Radio Shack announced a clearance price of $649.95 until stock ran out; the 4P was then discontinued.

Tandy later built the 4P with a gate array main board, sold as catalogue number 26-1080A. One service manual covers both boards.

Specifications

TRS-80 Model 4P (Tandy service manual, section II)

Item	Detail
CPU	Z80A, 4 MHz (2 MHz in Model III mode)
Memory	64 KB RAM, expandable to 128 KB; 4 KB boot ROM; 2 KB video memory
Keyboard	70 keys including a 12-key numeric pad, BREAK, CTRL, CAPS, CLEAR and F1-F3
Display	9-inch black and white CRT; 64 or 32 x 16 in Model III mode, 80 or 40 x 24 in Model 4 mode
Disk drives	Two single-sided double-density 5.25-inch half-height drives, 184 KB each
Interfaces	I/O bus, RS-232-C (DB-25), parallel printer (34-pin edge)
Options	640 x 240 graphics board; 300 baud auto-answer modem board
Power	105-130 V AC 60 Hz (US), 240 V 50 Hz (Australia), 220 V 50 Hz (Europe); 1.5 A typical, 1.7 A maximum
Operating temperature	13-27 °C (55-80 °F)
Size and weight	9.3 in high, 16.5 in wide, 13.25 in deep; 26 lb

Source: Tandy service manual, pp. 3 and 9. The 4P has no cassette interface and no connector for external floppy drives. In Model III mode, cassette output goes to the internal speaker.

Hardware

Boot ROM

The desktop Model 4 has the Model III BASIC in ROM. The 4P has only a small boot ROM, which can load an image of Model III BASIC (normally the file MODELA/III) from disk so that Model III programs run. The boot ROM can also boot from a hard drive or over the RS-232 port. Holding the full stop key while pressing RESET runs the boot ROM's memory test. Tandy's troubleshooting procedure boots the machine from a TRSDOS 6.1.1 or later system disk.

Main logic board

The main logic board lies flat in a metal pan at the bottom of the chassis. Two boards exist:

• On the original board, PCB 8709427, the video circuit makes the 12.672 MHz clock for the 80 x 24 modes with an NE564 phase-locked loop (U148) tuned by a trimmer capacitor.
• The gate array board puts most of the timing logic in U148 and makes the 12.672 MHz clock with a PLL multiplier module, U146, so it has no trimmer to set.

On both boards the floppy controller and RS-232-C circuits are part of the main board.

Power supply

The 4P uses Tandy's 65 W flyback switching supply, assembly 8790049, mounted under the top cover with a mylar insulator between them. It delivers +5.05 V, +12 V for the CRT, +12 V for the drives and −12 V, switching at about 40 kHz, with a crowbar on the +5 V output that trips between 5.8 and 6.8 V. The same supply is listed for the desktop Model 4.

Display

The 9-inch display was built in two versions with their own parts lists: 8790612 with a black and white tube and 8790613 with a green tube. The display board takes +12 V and draws about 0.85 A; it has its own 2 A fuse.

Disk drives

The two half-height Tandon TM50-series drives sit in one assembly at the right of the screen, drive 0 on the left and drive 1 on the right; the parts list gives the single-sided TM50-1.

Known faults

• The mains filter capacitors in the power supply can fail with a pop and a puff of smoke at switch-on; the trs-80.com repair notes name C32 and C33 on Tandy's own supply. The TRS-80 Model 4P Capacitor Replacement Guide lists them.
• A supply that ticks and starts nothing is in overload shutdown; on the 4P the short is usually in a disk drive.
• Tandy issued mandatory bulletins for the 4P on power supply current limiting (4P:4), AC harness problems (4P:13), a board layout error on revision A boards (4P:10) and lock-ups in 4 MHz mode (4P:15).

See the TRS-80 Model 4P Troubleshooting Guide for symptoms and checks.

Documentation

• Radio Shack TRS-80 Model 4P Service Manual: Tandy's 1984 service manual for the original and gate array 4P, with schematics and parts lists for the main boards, the 65 W power supply and both versions of the display.

Related pages

• TRS-80 Model 4P General Maintenance
• TRS-80 Model 4P Troubleshooting Guide
• TRS-80 Model 4P Capacitor Replacement Guide
• TRS-80 Model 4
• TRS-80 Model 4D
• CRT Discharge Procedure