BBC Micro Model B
A 2 MHz 6502 with Acorn ULAs, a 6845 and a stack of IDC headers underneath; the CRTC refreshes the DRAM, so a dead CRTC looks like a dead machine.
Open the BBC Micro Model B in Retro Bench- Also
- BBC Microcomputer Model B, board issues 1-7
- Year
- 1981
- CPU
- 6502A at 2 MHz; slowed to 1 MHz for the VIAs, 6845, ACIA, SERPROC, ADC and 1 MHz bus
- RAM
- 32 KB: 16 x 4816 16K x 1 DRAM in two banks (CAS 0, CAS 1)
- ROM
- 16 KB MOS (IC51) + 16 KB BASIC (IC52); four sideways sockets, 16 ROM numbers decoded
- Video
- Video ULA + 6845: MODEs 0-6 bitmapped (up to 640 x 256), MODE 7 teletext by SAA5050; RGB, composite, UHF
- Sound
- SN76489: three tones plus noise; optional TMS5220 speech
- OS
- MOS 1.2, BBC BASIC II
Chips
| Chip | Where | Role |
|---|---|---|
| 6502 / 65C02 CPU | IC1 (6502A) | CPU |
| Video ULA (Ferranti 5C094 / VLSI VC2069 VIDPROC) | IC6 | Serializes screen bytes into RGB through a 16-entry palette, mixes in teletext, draws the cursor and divides 16 MHz into 8, 4, 2 and 1 MHz. |
| MC6845 CRT controller | IC2 | Makes sync, display enable, cursor and the video RAM refresh addresses. |
| Mullard SAA5050 teletext character generator | IC5 | Teletext character generator: turns 7-bit character codes into rounded characters, colors and block graphics. |
| 6522 VIA | IC3 (system VIA), IC69 (user VIA) | Two 8-bit ports, timers, serial and user port |
| Serial ULA (Ferranti 2C199 / VLSI VC2026 SERPROC) | IC7 | Baud rate generators for the 6850, switching between cassette and RS423, cassette data separator, tone synthesis and motor control. |
| MC6850 asynchronous comms adapter | IC4 | Asynchronous serial interface (ACIA). |
| SN76489 / SN94624 (TMS9919) sound generator | IC18 | Three tone voices and one noise voice, written over an 8-bit data bus. |
| NEC uPD7002 4-channel A/D converter | IC73 | Four-channel integrating A/D converter, 8 or 10-bit, with a microprocessor bus interface. |
| Intel 8271 floppy disk controller | IC78 (disk upgrade) | Single-density (FM) floppy disk controller with internal CRC, seek and head positioning. |
| WD1770 / WD1772 floppy disk controller | 1770 upgrade daughter board (replaces the 8271) | Single and double density floppy disk controller with built-in data separator and write precompensation. |
| MC68B54 advanced data link controller (ADLC) | IC89 (Econet upgrade) | Advanced data link controller: HDLC/SDLC framing, flag detection, CRC and FIFOs. |
| 4816 16K x 1 dynamic RAM (+5 V only) | IC53-IC68 (bank CAS 0 and CAS 1) | 16K x 1 dynamic RAM, single +5 V supply. |
| 27128-pinout 16K x 8 ROM | IC51 (MOS), IC52 (BASIC), IC100, IC101 (sideways) | 16K x 8 ROM or EPROM in the JEDEC 27128 pinout. |
| 2764-pinout 8K x 8 ROM | IC88 (8K DFS/DNFS EPROM when fitted) | 8K x 8 ROM or EPROM in the JEDEC 2764 pinout. |
| 74LS138 3-to-8 decoder | IC24 | 3-to-8 line address decoder. |
| 7404 / 74S04 hex inverter | IC43 | Six inverters; often used, with a crystal, as a clock oscillator |
Ports and connectors
- UHF out (TV), Phono socket, UHF channel 36 PAL
- Video out (BNC, SK2), BNC socket: center and shell
- RGB (SK3), DIN-6 240° socket
- RS423 serial (SK4), 5-pin 'domino' DIN socket, pins A-E
- Cassette (SK5), DIN-7 270° socket
- Analog in (SK6), DA-15 socket (15-way D-type)
- Econet (SK7), DIN-5 180° socket
- Disk drive (PL8), 34-way IDC header under the case
- Printer (PL9), 26-way IDC header under the case
- User port (PL10), 20-way IDC header under the case
- 1 MHz bus (PL11), 34-way IDC header under the front edge
- Tube (PL12), 40-way IDC header under the case
- Keyboard (PL13), 17-way single-row header (keyboard ribbon) UNVERIFIED
- Power supply to main board (7 push-on leads), Seven 0.25 in push-on (Faston) tabs spread over the board UNVERIFIED
- Auxiliary power out (disk drives), 6-way AMP socket under the case (mates with AMP 1-350234-9) UNVERIFIED
Power rails
| Rail | Expect | Measure at |
|---|---|---|
| +5 V | +5 V | 6502 pin 8; VIDPROC pin 16; SERPROC pin 28; 7002 pin 14; 4816 pin 8 |
| -5 V | -5 V | PSU -5 V lead; RS423 driver DS3691 (IC75); LM324 cassette stages |
| +12 V | +12 V | Auxiliary power socket |
| Video ULA reference | Three diode drops (D16-D18) above 0 V UNVERIFIED | VIDPROC pin 15 |
| ADC reference | About 1.8 V | 7002 pin 8 to 7002 pin 9 |
Symptoms
- Dead: no beep, no picture: check 6502, VIDPROC, 6845, VIA
- Boots and displays but no sound, no tape, no RS423: check SN76489, SERPROC
- Picture flickers or sound on vision when a power lead is flicked
- Twinkling characters or smeared cursor in MODEs 0-2, appearing after months of use: check VIDPROC
- Twinkling only in MODE 7, or odd crashes in certain games, after an A to B upgrade: check 4816, 6502
- Memory faults; suspect one RAM bank: check 4816
- Tape will not load: check SERPROC, ACIA
- Keys on the right-hand side dead
- Hiss or digital noise on the sound (issues 1-3): check SN76489
- MODE 7 blank or broken, other modes fine: check SAA5050
- RS423 printing stops after a few characters: check SERPROC
- Hiss, crackle and white acrid smoke from the power supply