RCA UT5 ROM and Microterminal

Last updated Aug 13 2018. Written by Herb Johnson, (c) Herb Johnson, except for content quoted extensively from others. This is a one of a series of Web pages supporting the COSMAC 1802 microprocessor, as used in the 1802 Membership Card kit. Go to the linked Web page for details.

In the ZIP file

The UT5 ROM ZIP file at this Web link contains a hex file and hand-disassembled 1802 ASM source, and some documentation, as described below. Thanks to Robert Coward for the disassembly and discussion and some photos. Thanks to Loren Christensen for information on a similar RCA product.


Among many COSMAC 1802 microcomputers, RCA produced two that look similar and include a calculator as a user device. This Web page describes the CDP18S020 and CDP18S030, and provides details of the CDP18S021 Microterminal on the '020 product, under the control of the UT5 ROM monitor. Documents, source and hex listings are provided for UT5 and the calculator hardware. - Herb

CDP18S020 and MicroTerminal

[RCA Microterminal]

In 1977, RCA produced the CDP18S021 Microterminal and the UT5 ROM monitor, as a means of operating an 1802 in a simple way. The Microterminal is a calculator keypad and display, with additional CMOS logic controlled by an 1802 microprocessor. RCA also produced an 1802 computer board which supported the '021 Microterminal, as the RCA CDP18S020 Evaluation Kit. The Microterminal product and interface, is discussed in the publication "Instruction Manual for RCA COSMAC Microterminal, MPM-212". A scanned PDF copy of MPM-212 is available at this Web link. The '020 Evaluation Kit is discussed in "Evaluation Kit Manual for the RCA CDP1802 COSMAC Microprocessor", RCA publication MPM-203.

As part of the MPM-212 manual is a user's description, source listing, and hex dump of the UT5 ROM. RCA produced that ROM as CDPR522, a masked-ROM version of the CDP1832D. The UT5 code operates the calculator interface to accept and display hexidecimal values and monitor commands. The MPM-212 manual also describes the interface circuits, contained inside the calculator case. Robert Coward obtained a '021 Microterminal, and disassembled the UT5 hex file, to modify UT5 amd operate the Microterminal on another 1802 system. He kindly permitted me to distribute that work, and provided the '021 photo.

UT5 is similar in function to another RCA ROM monitor, UT4, which I describe on another Web page.

Similar but not the same: CDP18S030 "Micromonitor"

[RCA Micromonitor] [RCA Micromonitor]

RCA also produced a similar calculator-based product (left), as part of another evaluation kit from 1978, the RCA CDP18S030 Micromonitor System (right), which is a large 1802 single-board system contained in a small suitcase. As explained to me by Loren Christensen, who has this system: "The Micromonitor is an in-circuit debugger that gets interposed between the 1802 and the system under test, (i.e. one step better than an in-circuit emulator). It's controlled either by the provided handheld interface, or by a serial terminal. There are full instructions, and what appear to be complete schematics, in the manual "Instruction Manual for the RCA COSMAC MIcromonitor CDP18S030, MPM-218".

The ROMs in the '030 product total 6 kbytes, and contain a much more elaborate operating environment for the 1802 than UT5. Additional RCA products support a diskette based operating system. There's no"ROM dump" or source code in the MPM-218 manual. Loren created a PDF scan of the MPM-218 manual, and provided the '030 photos shown

At my first glance, the two calculator-based "terminal" devices looked the same, but they are not. As explained to me by Robert: "The RCA CDP18S030 system has something resembling the CDP18S021 Microterminal, bolted down to the PCB. Though both are a calculator case with keyboard and display, the number of buttons are different. The CDP18S021 Microterminal contains its own electronics, and is connected via a ribbon cable. The CDP18S030 device may be a passive keypad/LED display unit, with the driver electronics on the main PCB. The chips immediately surrounding it on the large board, look pretty similar to what is inside the CDP18S021 device."

- Herb Johnson

Other Related Web pages

I have some dates for and documents about 1802 development on this Web page.

The A18 MS-DOS 16 bit/32-bit cross assembler for the 1802.

The RCA UT4 ROM monitor.

UT20 ROM monitor.

- Herb Johnson

Contact information:
Herb Johnson
New Jersey, USA
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This page and edited content is copyright Herb Johnson (c) 2018. Copyright of other contents beyond brief quotes, is held by those authors. Contact Herb at www.retrotechnology.com, an email address is available on that page..