Omron SK20 MiniLSS "Couldn't read LR-sizes" Communication Timeout: Cable, COM Port, and DOS Fix

The error Couldn't read LR-sizes followed by Communication time out when launching MiniLSS 1.x against an Omron mini SK20 PLC is almost always caused by one of three things: a wrong SP16 peripheral cable pinout, mismatched COM port settings, or MiniLSS failing to hold the UART while Windows 95 is multitasking. This article walks through each of those failure modes in the order they are most likely to be the culprit, with the exact DOS boot-disk procedure, COM1 verification steps, and a verification matrix to confirm a working link.

1. Problem Statement

Reported by the field user (Windows 95 laptop, Fujitsu LifeBook 400 series, Pentium MMX 166/200 MHz, 48 MB RAM, COM1, MiniLSS 1.1 then 1.2, hand-built cable to mini SP16):

1. MiniLSS starts.
2. Software attempts to read the PLC's Link Relay (LR) area size at connection time.
3. Dialog displays: Couldn't read LR-sizes and then Communication time out.
4. Slowing the laptop CPU via CPUKILLER / similar utility does not change the result.

Because the failure happens at the very first read, MiniLSS never establishes a host-link session. The LR-size request is the first structured command the editor issues over the peripheral bus, which makes it an excellent built-in cable / port / settings canary.

2. Affected Hardware and Software

3. Root Cause Analysis

Three root causes must be ruled out, in this order, because they are ordered by probability and by how cheaply they can be verified:

1. Wrong / hand-reversed cable pinout (most likely). The user built a cable to the SP16 themselves. A single crossed line, a missing ground, or a swapped handshake is enough to make every host-link command time out. MiniLSS reports this generically as LR-sizes read failure.
2. COM1 is not actually COM1, or the UART address is non-standard. Some Lifebook 400 sub-models route COM1 through a flexible bay docking connector or assign it to a non-default IRQ. MiniLSS 1.x uses BIOS enumeration only; it does not scan PnP.
3. Windows 95 owns COM1. Even with a correct cable, the Windows 95 COM driver buffers bytes and intercepts interrupts. MiniLSS 1.x talks directly to the 16550 UART at port 0x3F8. Windows 95 may not have placed the port at that address, or may have left a VxD holding the line.

The CPU throttling step is unrelated to baud-rate generation on a 16550 UART; it can be removed from the workflow.

4. Required Materials

• Omron SP16 programming console (or direct SK20 peripheral port breakout).
• DB-9 male to SP16 peripheral connector cable, verified against the pinout in section 5.
• Working COM1 with confirmed 0x3F8 base address (built-in or pure-ISA card; PCMCIA serial cards are not reliable under DOS).
• MS-DOS 6.22 boot floppy (or FreeDOS 1.x boot floppy).
• MiniLSS 1.2 installation files copied to a second DOS-readable medium (hard-drive partition, second floppy, or DOS-mode USB stick if BIOS supports it).
• Multimeter with continuity mode for cable verification.

5. SP16 Cable Pinout and Wiring

The mini SP16 peripheral connector is a round mini-DIN style connector on the SP16 handheld that exposes the SK20's peripheral bus. The PC end is a standard DB-9 female mating with COM1. The exact pin assignment varies by SP16 hardware revision; the table below is the documented host-link-style cross-over most SP-series cables implement. Verify every line with a continuity tester before powering the SP16; pin numbering is not standardized across early Omron accessories.

Wiring rules specific to this case:

• SD ↔ RD must be a cross (2 ↔ 3). A straight-through cable is the single most common cause of the LR-sizes timeout.
• RS ↔ CS is also a cross (7 ↔ 8). Some hand-built cables omit the handshake and loop RTS to CTS at the PC end; this sometimes works on slow baud rates, but is unreliable at 19 200.
• SG must be connected. A floating ground produces intermittent timeout exactly like the user's symptom.
• FG (shield / drain) must be bonded at the PC chassis end only. Bonding at both ends creates a ground loop that biases the RS-232 thresholds and corrupts the LR-size read.

6. COM Port Configuration

MiniLSS 1.x does not provide a friendly COM-port dialog. It auto-detects COM1 at 0x3F8, IRQ 4. If the laptop's BIOS has remapped COM1 (some Lifebook docking configurations and some APM-aware BIOS revisions do this to free IRQ 4 for a docking-station serial chip), MiniLSS will still attempt 0x3F8 and fail to read the LR area.

6.1 Confirm the UART address from a clean DOS prompt

1. Boot the DOS 6.22 floppy (procedure in section 7).
2. At the A:\> prompt, run the DOS debug command to read the BIOS data area at 0x0040:0000, where COM port base addresses are stored:

A:\>DEBUG
-D 0040:0000 L0008

Expected output (16-bit little-endian, COM1 = 0x03F8, COM2 = 0x02F8, COM3 = 0x03E8, COM4 = 0x02E8):

If the first two bytes are not F8 03, COM1 is remapped. MiniLSS will not find the SK20.

6.2 Confirm the UART responds

1. From the same DEBUG session, type:

This reads the Receiver Buffer Register of COM1. A successful read returns a byte and increments the IP. If the system hangs or returns FF repeatedly, the UART is not at 0x3F8 or the chip is not decoding the I/O cycle.

6.3 Set the port parameters outside MiniLSS

MiniLSS sets 9 600 7-E-2 internally for the SP-series peripheral bus. If you want to verify the port with a terminal first, run a plain DOS terminal program (a shareware COMTEST or the FreeDOS CTTY) at:

Then issue the Omron host-link @00RD test frame at 9 600, 7, E, 2 from the terminal. The PLC will not echo in 7-E-2; the verification is that no bytes are lost and no overrun errors appear. If you see overrun or framing errors, the cable is the problem even if MiniLSS is configured correctly.

7. DOS Boot Disk Procedure

MiniLSS is a real-mode DOS executable. It cannot share the COM port with the Windows 95 VCOMM driver. Booting DOS 6.22 directly (not a Windows 95 DOS box, not an MS-DOS-mode shortcut) is the only supported runtime.

7.1 Create an MS-DOS 6.22 boot disk from a Windows 95 host

1. Insert a 1.44 MB floppy in drive A:.
2. Open Control Panel → Add/Remove Programs → Startup Disk tab.
3. Click Create Disk. Windows 95 builds a generic MS-DOS 6.22 boot floppy with IO.SYS, MSDOS.SYS, COMMAND.COM, FDISK.EXE, FORMAT.EXE, EDIT.COM, SYS.COM, and a minimal CONFIG.SYS / AUTOEXEC.BAT.
4. Verify the disk boots. Leave the disk in drive A: and reset the laptop. The system must boot to A:\> with no Windows 95 banner.

7.2 Create an MS-DOS 6.22 boot disk from a modern Windows host

1. Download a known-good MS-DOS 6.22 boot-image .img (or extract the Win9x OEM boot disk files from any Windows 98 SE recovery partition).
2. Use Rufus in FreeDOS or DD Image mode to write the image to a 1.44 MB floppy or, more practically today, to a USB stick that the Lifebook BIOS can boot (most 400-series Lifebooks cannot boot USB; if yours cannot, use a USB floppy drive such as the Toshiba PA3109U-1FDD or a Y-E Data clone).

7.3 Prepare the MiniLSS media

1. Copy the MiniLSS 1.2 folder structure to a second DOS-formatted floppy, or to a small FAT16 partition on the hard drive that the DOS boot does not mount as a Windows drive letter.
2. Edit the boot floppy's CONFIG.SYS so it contains only:

FILES=30
BUFFERS=20
DEVICE=C:\DOS\HIMEM.SYS /TESTMEM:OFF
DOS=HIGH,UMB

Leave CONFIG.SYS free of any EMM386, RAMDRIVE, or third-party CD-ROM drivers. They re-map COM I/O and will silently break MiniLSS even if the cable and port are correct.

3. Set AUTOEXEC.BAT to:

@ECHO OFF
PROMPT $P$G
PATH=C:\MINILSS;

Then C:\MINILSS\MINILSS.EXE is reached from any drive letter.

8. MiniLSS Configuration

On a clean DOS session with the verified cable connected and the SP16 docked on the SK20:

1. Power the SK20 first; then connect the SP16; then launch MiniLSS.
2. In MiniLSS, the online editor opens directly against the connected PLC when started with the /ONLINE switch:

3. MiniLSS issues the @00RD family of host-link frames to enumerate the PLC's memory layout. The first read is the LR area size. A successful read returns the LR word count and the dialog advances. A failed read produces exactly the Couldn't read LR-sizes / Communication time out pair the user is seeing.

9. Step-by-Step Connection Procedure

1. Power off the SK20.
2. Connect the SP16 to the SK20 using the SP16's own captive cable.
3. Connect the SP16's peripheral output to the PC's COM1 using the cable from section 5.
4. Boot the DOS 6.22 floppy. Confirm A:\> prompt with no Windows banner.
5. From DEBUG, confirm COM1 = 0x3F8 (section 6.1).
6. Power on the SK20. Wait for the SP16 to display READY or similar.
7. Run MINILSS /ONLINE.
8. If MiniLSS displays LR-sizes timeout, go to section 10.
9. If MiniLSS displays the project tree, the connection is good. Save the project, then power down in reverse order.

10. Troubleshooting Matrix

11. Memory Area Reference for the LR Read

The LR-sizes request is a host-link frame that asks the SK20 how many Link Relay (LR) words it has. The SK20 LR area is a shared data region used in multi-PLC link groups; in a stand-alone SK20 it is still allocated and MiniLSS expects a non-zero size reply. If the SK20 returns 0 (LR area disabled in PLC setup) the editor may still report a timeout on early 1.x builds, so verify DM area word DM 6600 through DM 6614 in the SK20 setup (the standard Omron mini PLC default area) for the LR enable bits before declaring the link healthy.

For context, the SK20 standard area map is:

12. Verification Checklist

• Cable passes continuity test on every signal and SG.
• Shield bonded to PC shell only, isolated from signal ground.
• Lifebook is undocked; using native COM1.
• DEBUG -D 0040:0000 L8 shows COM1 = F8 03 00 00.
• DEBUG -I 3F8 returns a valid byte.
• CONFIG.SYS has no EMM386 and no third-party COM drivers.
• CPUKILLER / throttler removed.
• MiniLSS 1.2 launched with /ONLINE from a clean A:\>.
• LR-size read returns a non-zero count and the project tree opens.
• A read of an IR bit (e.g. IR 000.00) round-trips in the online monitor.

13. Safety and ESD Notes for SK20 Work

The SK20 is an AC-input, relay-output PLC. Before seating or unseating the SP16, or hot-swapping the peripheral cable, kill the SK20's main breaker. The SP16 connector is not hot-pluggable in the field; the host-link transceiver on the SK20 can be damaged by an in-rush spike on the SP16's 5 V rail. ESD precautions (wrist strap, grounded mat) are appropriate when the SK20 is open on the bench.