DL11 asynchronous serial line interface

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).

Configuration

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

Versions

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.