DHU11 asynchronous serial line interface

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DHU11 M3105 board

The DHU11 asynchronous serial line interface was a replacement for the UNIBUS DH11. It was not programmatically exactly identical to the DH11, but had all the same basic capabilities: it provided 16 RS-232 asynchronous serial lines, and used DMA on the output side.

The reasons for the replacement of the DH11 are not known, but likely the major one was cost: the DH11 consisted of an entire double system unit backplane, containing multiple cards; the DHU11 was a single card. It had some minor improvements over the DH11 (including a fix for its silo problem), but they seem not significant enough by themselves to warrant its replacement. They included:

  • input used a 256-character FIFO buffer (somewhat larger than the original DH11's 64-character one);
  • the DHU11 includes several microprocessors, and has extended capabilities, including a diagnostic self-test, and flow control support;
  • the DHU11 has the ability to transmit under program control; each line has a 64-character FIFO output buffer.


H3029 Distribution Panel

Physically, the DHV11 consisted of a single hex card, the M3105, which plugged into a MUD slot. It connected to a pair of H3029 distribution panels (8 lines each) via BC05L flat cables, two per distribution panel; the BC05L connects to a 40-pin Berg header on each end. The H3029 contains eight male DB-25 connectors (DB-25P).

Device registers

The registers in the DHU11 are somewhat different from those in the DH11:

  • it was intended to be program compatible with its QBUS equivalents (the DHV11; and later the DHQ11, which could be set to exactly emulate the DHU11), and since the QBUS has a larger address space than the UNIBUS, it needed two word-sized registers to hold the output buffer address;
  • the previously mentioned improvements.
Register Abbreviation Address
Control and Status Register CSR 760020
Receive Buffer Register RBUF 760022
Receive Timer Register* RXT 760022
Line Parameter Register LPR 760024
Transmit FIFO Data Register TFD 760026
Transmit FIFO Size Register* TFS 760026
Line Status Register* STAT 760027
Line Control Register CTRL 760030
Transmit Buffer Address Register Low TBADL 760032
Transmit Buffer Address Register Hi TBADH 760034
Transmit Buffer Count Register TBCT 760036

Registers marked with a '*' are byte width; the others are all word-width. The RBUF and RXT share an address, as do the TFD and TFS: the RBUF is read only, and the RXT is write only; the TFD is write only, and the TFS and STAT are read only. The LPR through TBCT are replicated, with one for each line in the device; the 'Line' field in the CSR indicates which one is currently available.

The addresses shown are for the first DHU11 in a system; additional ones are normally set to be at 760040, 760060, etc.

Control and Status Register (CSR)

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
  • TXA - Transmitter Action
  • TXIE - Transmit Interrupt Enable
  • DF - Diagnostic Fail
  • TDE - Transmit DMA Error
  • Tx Line - Line which set TXA
  • RDA - Received Data Available
  • RXIE - Receiver Interrupt Enable
  • MR - Master Reset
  • SKIP - Skip Self-Test
  • Line - Select LPR through TBCT

Line Parameter Register (LPR)

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Line Status Register (STAT)

DSR Unused RI DCD CTS Unused DHU Not Present
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Line Control Register (CTRL)

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Further reading

  • DHU11 Technical Manual , EK-DHU11-TM-001 (not yet online - it's bound, so a copy may have to be sacrificed to scan it)
  • DHUll Field Maintenance Print Set, MP-01794-01 (not yet online)

External links