Interfacing an ARL plasma spectrometer to an HP1000 minicomputer
R.C. Bigelow · Antarctica A Keystone in a Changing World · 1982
This paper describes a digital data link interface between an Applied Reasearch Labortories (ARL) 137000 Inductively Coupled Plasna emission spectrometer and a Hewlett Packard HPiOOO niniconputer.The ARL is a self contained automated system that Measures the concentration of up to 32 elements in liquid samples.A dedicated Digital Equipment Corp. (DEC) PDPii/04 «inico«puter, or processor, controls the spectrometer, records the measured data on floppy disks, computes the element concentrations, and prints the results on an operator's terminal.A dialect of BASIC, written by the ARL staff, serves as both the programming language and operating system for the P0P controller.The HPiOOO is a general purpose, multi-tasking, interrupt-driven, data acquisition system that is programmed in FORTRAN or Assembly Language and runs under RTE-IMB operating system with Multi-Terminal Monitor (MTM).The digital data link eliminates manually keypunching and verifying the ARL data for entry into a data base on the USGS MULTICS system.Instead, data pastes from the ARL over the data link to the HPiOOO which reformats and stores it on 9-track magnetic tape for transport to MULTICS.The concept behind the data link is to have each minicomputer act as if the other is an intelligent terminal communicating at 1200 baud (120 characters per second) over standard, RS-232, hard-wired interfaces.This approach simplifies the hardware installation but creates some problems that mist be solved by 2 software.First, coordinating programs Must execute simultaniously in each processor to insure that the data is transmitted and received in proper seqtence; second, at a more elementary level, each processor Must control the rate at which it receives transmissions from the other.When a host computer comminicates with non-intelligent terminal!, it usually assumes that they are always listening for its output and that they can wait, if necessary, vntil the computer is ready to accept their input.Neither assumption is necessarily true when the 'terminal' is intelligent or is another computer.To avoid information loss when two computers communicate, each must request permission of the other to transmit data and then wait for a go-ahead signal to insure that the recipient is ready to recieve more data.This process of controlled interchange, called 'handshaking", solves the eleventry timing problem} it is written into software for each processor.When the HP1880 wishes to transmit over the data link, it first sends ENQ it overwrites the previous character in the buffer.The PDP nust intercept each incoming character, test it to determine whether it is part of a handshake sequence, store it in the PDP's neftorv, and be in position to intercept the next character before that character is overwritten.This constraint gives rise to tine-critical code in the PDP software. DESCRIPTION OF DATA LINK SOFTWARE:The software part of the data link consists of three user programs ("HP"/ICP% and "FSEND11 ) and a driver for the i2966A board ("DMX85").The driver is specialized svsten software that controls the 129666 board) it takes standard systew input, output, and control calls and inplenents then as well as handling input/output (I/O) protocols like the handshaking routine.It also defines the special characters and break characters for the board.Program "HP" runs on the PDP and enables its terminal (DEC terminal) on the ARL svste* to enilate a user terminal on the HPiOIO.Kith "HP" executing, the ARL operator can run the "ICP" progran on the HPilOS.Programs "ICP" and "FSEND" coordinate to transfer data fron the the ARL to the HPiOOO.Program "ICP* signals the PDP to load and run "FSEND" and then waits for input.Program FSEND' retrieves emission data fron a file stored on a floppy disk, calculates concentrations in parts per Million and transmits the results over the data link along with header information that identifies the data.The