DL11 asynchronous serial line interface

From Computer History Wiki
Jump to: navigation, search

The DL11 asynchronous serial line interface card (M7800) was a UNIBUS card for the PDP-11 series of computers; it was a quad format card, and used an SPC slot.

It provided both 20mA and (in some versions) EIA connectivity at speeds of 110 to 9600 baud (the range depended on the version); the DL11-E version also provided full modem control. Line connection was via a 40-pin Berg header on the top edge of the card, using the standard DEC asynchronous serial line pinout.

It was a functionally identical single-board replacement for the KL11 asynchronous serial line interface, which consisted of a dual card, and two standard single card FLIP CHIPs.

The TU58 tape drive is connected to a DL11-type device, so the device address, etc for TU58's are as for DL11's - the register addresses and interrupt vectors are those given below for the 'first' DL11 (DL11 #1).

Programing interface

The DL11 has 2 device registers for the receive side (one Control and Status Register - CSR - and one data buffer register), and similarly, two for the transmit side. Each DL11 has two interrupt vectors - again, one receive, and one transmit.

For the PDP-11 main console (which is always a KL11/DL11 compatible device), the 'zeroth' DL11, the standard is that 0777560 is the base address (so the receiver registers are 0777560-2, and the transmitter are 0777564-6), and 060 is the base vector.

The first DL11 after the console is always assigned the address 0776500, and vector 0300. Additional DL11's are assigned addresses and vectors immediately following, for DL11's #1-#16 (i.e. 0776500-676 and 0300-0476).


The bus address, interrupt vector, and line parameters (parity, stop bits, and data bits) were selected by soldered jumpers; the baud rate was set by a combination of a replaceable crystal (several different ones were standard), and a pair of 10-position dial selectors (one for transmit, one for receive) which selected a rate:

Crystal Speed 1 2 3 4 5 6 7 8
884.8 KHz 35.7 55 110 220 440 880 1320 1760
1.033 MHz 44.8 67.3 134.5 269 538 1076 1614 2152
1.152 MHz 50 75 150 300 600 1200 1800 2400
4.608 MHz 200 300 600 1200 2400 4800 7200 9600

(The 9th and 10th dial selector positions selected external clock inputs, not divisions of the crystal.)

The address and vector jumpers specify the base of the register bus addresses, and vectors, in binary. On the DL11, it is only possible to set the base of the entire group of 4 registers; the transmit and receive register groups cannot be moved around independently. Similarly for the vector, only the base of the pair of vectors can be configured; the receive (at B) and transmit (at B+4) are paired in the hardware.

The simple rule on the DL11 is that vector jumpers are the inverse of address jumpers: for the vector, jumpers are 'in' for '1', and for the address, they are 'in' for '0'. (This inversion can be a memory jogger - 'Oh, the DL11, that blasted interface where the jumper sense is inverted between address and vector!' Then one only has to look up one of the two.)

Address jumper A3 is the 3rd bit of the address, i.e. the 010 bit, the lowest one that is configurable (the device is a block of 8 bytes, from xxxxx0 to xxxxx7, so it is not possible to set any of the low 3 bits in the base address, they must be 0), and A10 is the highest bit which can be changed. V3 through V8 perform a similar function for the vectors.

The following table shows the bits which are affected when the jumper is in.

Address Jumper Address Bit Vector Jumper Vector bit
A3 -010 V3 +010
A4 -020 V4 +020
A5 -040 V5 +040
A6 -0100 V6 +0100
A7 -0200 V7 +0200
A8 -0400 V8 +0400
A9 -01000
A10 -02000

Sample configurations

The following table gives the jumper configuration for the two most common DL11 configurations (main console, and 'first' DL11 - often a TU58, now):

Jumper Bit Console First DL11
A3 010 In In
A4 020 Out In
A5 040 Out In
A6 0100 Out Out
A7 0200 In In
A8 0400 Out Out
A9 01000 Out In
A10 02000 Out Out
V3 010 Out Out
V4 020 In Out
V5 040 In Out
V6 0100 Out In
V7 0200 Out In
V8 0400 Out Out


The DL11 came in 5 versions, -A through -E.

The -A and -C versions (board M7800-YA on at least one of these) were 20mA interface only, and omitted the components necessary for the EIA interface.

The -A and -B apparently did not provide full baud rate selection and selectable data coding (they were restricted to 8-bit data, no parity, and 1 or 2 stop bits); they also could not provide 'split' speeds (differing baud rates on transmit and receive). Apparently, these versions had more limited address selection capability; they could only be configured to 0776XX0, where XX ranged from 050 to 067.

Only the -C through -E could be ordered with the 4th (9600-baud capable) crystal (it is not known whether the earlier ones would work if this crystal were inserted). Only the -E provided full modem control.

The -A and -C versions are easy to identify visually; they are both missing a large number of components. How to tell the others apart (if, indeed, there is any difference between the -D and -E) is not yet documented.