Atari 800XL SIO Port Repair

The 13-pin serial I/O socket, marked "PERIPHERAL", at the left-hand end of the 800XL's rear panel

Disk drives, cassette recorders, printers and modems connect to the Atari 800XL through the 13-pin serial I/O (SIO) socket J1, which runs at up to 19,200 baud. POKEY (U22) sends and receives the serial data and clocks; the PIA (U23) handles the command, proceed and interrupt lines and switches the cassette motor through transistor Q7; and the socket also carries +5 V and an audio input. When a machine runs but cannot load from or save to any SIO device, or the recorder's motor never starts, Sams' service data gives the checks for each line, and Atari's checklist names the parts behind each SIO test failure.

Symptoms

• A known-good disk drive or recorder will not load, or will not save, on this computer.
• The cassette motor does not start when loading or saving. Sams points at the PIA (U23).
• Cassette save or load fails. Sams points at POKEY (U22).
• Audio from the cassette's audio track is not heard. Sams names C19 and C110; see Atari 800XL Sound Repair.

Check the cable and the peripheral on another computer first. A peripheral that needs its own +5 V from the socket also relies on pin 10.

Safety

Pin 10 carries the computer's +5 V. Do not short it to the ground pins 4 and 6 while probing.

Tools and parts

• A logic probe, an oscilloscope and a meter.
• An Atari program recorder and a blank tape, or a disk drive, for testing.
• Replacement parts as found: POKEY (Atari C012294), the PIA (C014795), the MPSA55 transistor for Q7 (C014809), 100 Ω resistors for R128-R131 and the 13-pin socket (C012995).

Atari's figure 1-9: the SIO socket's pin numbering and signals

SIO socket J1 (Atari figure 1-9 and schematic; Sams test points)

Pin	Signal	Board connection
1	Clock input	POKEY pin 26 through R129 (100 Ω)
2	Clock output	POKEY pin 27 through R128 (100 Ω)
3	Data input	POKEY pin 24 through R130 (100 Ω)
4, 6	Ground	–
5	Data output	POKEY pin 28 through R131 (100 Ω)
7	Command	PIA pin 19 (CB2)
8	Motor control	Q7 (MPSA55) through the bead L11; Q7 is driven from PIA pin 39 (CA2) through R91
9	Proceed	PIA pin 40 (CA1)
10	+5 V / ready	+5A through L10, with C79 (22 µF)
11	Audio input	To the audio amplifier through C19
12	Not connected	–
13	Interrupt	PIA pin 18 (CB1)

Procedure

Open the machine

1. Remove the six screws from the bottom cover, tip the top cover up from the left and disconnect the keyboard ground strap and the keyboard cable.
2. Remove the four board screws and lift the board out, pushing out the right side of the housing so the controller ports clear it. The shield is held together by ten screws.

Cannot save

Sams' check, using a program recorder:

1. Type a short program, or load one. Type CSAVE and press RETURN; after two beeps, press PLAY and RECORD on the recorder and press RETURN again.
2. While it saves, check for waveforms on pins 26, 27 and 28 of POKEY (U22).
3. If any of them is missing, check U22 by substitution.
4. If the waveforms are correct, check the connections at pins 1, 2 and 5 of J1.

Cannot load

1. Type CLOAD and press RETURN; after one beep, press PLAY on the recorder and press RETURN again.
2. Check for pulses at pin 24 of POKEY while it loads; pin 24 is POKEY's serial data input, fed from J1 pin 3.
3. Sams then directs the check to the connections at J1 pin 3. If there are no pulses at pin 24 while the recorder plays, follow the line back from pin 24 through R130 to J1 pin 3.

Recorder motor does not start

Atari's schematic of the SIO control lines: the PIA's CA2 output drives Q7, which switches +5A through L11 to the motor control pin, J1 pin 8

1. Type POKE 54018,52 and press RETURN. Put the logic probe on pin 39 of the PIA (U23): it should go from high to low, which starts the recorder motor.
2. If it stays high, check U23 by substitution.
3. If the PIA output changes correctly, check the switch transistor Q7 and the connections at J1 pin 8. On the schematic Q7's base is driven from pin 39 through R91 (2.7 kΩ), its emitter is on +5A, and its collector reaches J1 pin 8 through the bead L11.
4. Type POKE 54018,60 and press RETURN to stop the motor.
5. Atari's checklist for a failed motor-control voltage test names Q7, L11 and U2.

Other SIO test failures

Atari's field test used the SuperSALT cartridge and its test assembly, which loop the SIO lines back. Its checklist gives these parts for each failure:

• SIO port: U22, U23 and C75-C78.
• Two-way clock: J1 and U22.
• External audio: J1 and U1.

Testing

1. Save a short program to tape and load it back, or boot a disk.
2. Check +5 V on J1 pin 10 against ground on pin 4.
3. Run the POKE commands above and check that the recorder motor starts and stops.

Related pages

• Atari 800XL
• Atari 800XL Troubleshooting Guide
• Atari 800XL Sound Repair
• Atari 800XL Controller Port Repair
• Sams Computerfacts Atari 800XL
• Atari 800XL Computer Field Service Manual