EPson Serial Protocol primer

Source: HX-20 Software; Assembler & Disassembler, p. 70 and up

The protocol is a modified subset of CP/NET. The original protocol assumes a simple structure consisting of a computer and a peripherial connected with an asynchronous line. As the Epson variant allows for more pheripherials, the ENQ message is extended with Source and Destination fields.

A typical exchange between a computer (HX-20, PX-8, PX-4) and a peripherial (disk unit or display unit) looks like this:

• ENQ PROCEDURE: initiated by the Initiator
• HEADING PROCEDURE: Initiator indicates the function
• SEND TEXT PROCEDURE: Initiator sends data
• HEADING PROCEDURE: Responder indicated function response data
• SEND TEXT PROCEDURE: Responder sends data
• EOT: Initiator and Responder both send an EOT

ENQ PROCEDURE

This has two phases;

• The enquiry from the Initiator,
• The acknowledgement from the Responder.

The enquiry consists of 5 characters:

1. EOT (End Transmission, to close previous message exchange)
2. P1 (00h for Select. The alternate value 80h for Polling is not supported)
3. DID Destination or Slave Identification
4. SID Source or Master Identification
5. ENQ

The acknowledgement from the Slave is a single character:

• ACK

Note that only the addressed Responder sends an Ack. Other Responders should stay passive until both devices have signalled the end of an exchange by sending an EOT.

HEADING PROCEDURE

This has three phases;

• The function code from Master to Slave,
• The acknowledgment from the Slave,
• The response from the Master.

The function code from Master to Slave contains 7 characters:

1. SOH 01h
2. FMT 00h for query, 01h for response
3. DID Destination Identification
4. SID Source Identification
5. FNC Function code
6. SIZ Text size (this appears to be one less than the number of data characters in the text block)
7. HCS Header checksum

The acknowledgment from the Slave is a single character:

• ACK when the function code message is received correctly and the function can be processed,
• WAK when the function code message is received correctly but the function cannot be processed,
• NAK when the function code message is not received correctly.

The response from the Master:

• when an ACK is received, continue with the SEND TEXT PROCEDURE,
• when a WAK is received, wait 100 milli-seconds and resend the function code message,
• when a NAK is received, resend the function code message,
• when another character is received, or no acknowledgment is received, restart with the ENQ PROCEDURE ??

SEND TEXT PROCEDURE

This has three phases;

• The payload from Master to Slave
• The response from the Slave
• The response from the Master

The payload from Master to Slave consists of a variable number of characters:

• 1. STX
• 1. data byte 0
• 1. data byte 1
• 1. …
• 1. Data byte N-2
• N+1 ETX
• N+2 CKS

The response from the Slave is a single character:

• ACK when the text is received correctly and can be processed,
• WAK when the text is received correctly but currently cannot be processed,
• NAK when the text isn’t received correctly.

The response from the Master:

• when an ACK is received, consider communication succesfully done,
• when a WAK is received, wait 100 milli-seconds and resend the payload, ??
• when a NAK is received, resend the payload. ??

When the Initiator is done the Header and Send Text procedure, the Responder starts its own Header and Send Text procedures

EOT

The Responder sends an EOT following the Send Text, which is confirmed by the Initiator with an Ack. The Initiator then sends its own EOT, which is confirmed by the Responder with an Ack.

Then the channel is free for the next message exchange.

Sample

Below a edited and annotated dump of a PX-4 “1>” requesting the first directory sector of a floppy drive “>2”. The original dump is produced by this Arduino sketch: https://github.com/electrickery/DualSerialMonitor.

#Serial scope v0.1.0
#Monitoring Serial1 as '1>'
#Monitoring Serial2 as '2>'

Enquiry (PX-4 to FD1)

1>04 31 31 23 05       # EOT, P1, FD1, PX-4, ENQ
2>06                   # ACK

Header

1>01 00 31 23 0D 00 9E # SOH, FMT, FD1, PX-4, RESET DISK, SIZE, HCS
2>06                   # ACK

Text

1>02 00 03 FB          # STX, 00, ETX, CKS
2>06                   # ACK

EOT (signals PX-4 is done talking)

1>04

Header (response message from FD1)

2>01 01 23 31 0D 00 9D # SOH, FMT, PX-4, FD1, RESET DISK, SIZE, HCS
1>06                   # ACK

Text

2>02 00 03 FB          # STX, 00, ETX, CKS
1>06                   # ACK

EOT (signals FD1 is done talking)

2>04

EOT (signals PX-4 is done talking)

1>04

Enquiry (PX-4 to FD1)

1>04 31 31 23 05       # EOT, P1, FD1, PX-4, ENQ
2>06                   # ACK

Header

1>01 00 31 23 77 02 32 # SOH, FMT, FD1, PX-4, READ SECTOR, SIZE, HCS
2>06                   # ACK

Text

1>02 01 04 01 03 F5    # STX, Drv, Trk, Sec, EOT, CKS
2>06                   # ACK

EOT (signals PX-4 is done talking)

1>04

Header (response message from FD1)

2>01 01 23 31 77 80 B3 # SOH, FMT, PX-4, FD1, READ SECTOR, SIZE, HCS
1>06                   # ACK

Text # STX, data00-data7F, RetCode, EOT, CKS

2>02 00 434F4E54454E54535458540076001C0102000000000000000000000000000000524541444D4520205458540030000703000000000000000000000000000000E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5 00 03 90
1>06                   # ACK

EOT (signals FD1 is done talking)

2>04

EOT (signals PX-4 is done talking)

1>04

ASCII Character codes

00h     NUL - Null (Send by PX at startup)
01h     SOH - Start Of Header
02h     STX - Start Of Text
03h     ETX - End Of Text
04h     EOT - End Of Text
05h     ENQ - Enquiry
06h     ACK - (Positive) Acknowledge
10h     WAK - Wait For Acknowledge (officially DLE)
15h     NAK - Negative Acknowledge

CP/NET field names (EPSP flavour)

CKS -  Text Checksum
Drv -  Disk drive (1 based)
FMT -  Message Format Code
HCS -  Header Checksum
Sec -  Sector
P1  -  Polling/Select function (only Select (00) is supported)
RetCode - Return code 
SIZE - Size of following text message
Trk -  Track

Physical interface

The TF-20 terminal floppy drive implements the network as two serial interfaces using a single UART. Data received on the Host port (HX-20 or PX-4/8) is routed to the Other peripherial port, data from the Other peripherial port is logically ORred with the Peripherial Tx signal. The PIN/POUT circuit using CTS and DTR is not used.

Sources

• HX-20 Software Assembler/Disassembler Manual, p.70-
• PX-4 Operating System Reference Manual, II-408
• TF-20 Technical Manual

fjkraan@electrickery.nl, 2024-03-31